High Coverage Water Aggregation and Dissociation on Fe(100): A

Article pubs.acs.org/JPCC

High Coverage Water Aggregation and Dissociation on Fe(100): A Computational Analysis Shaoli Liu,†,‡,§ Xinxin Tian,†,‡,§ Tao Wang,∥ Xiaodong Wen,†,‡ Yong-Wang Li,†,‡ Jianguo Wang,†,‡ and Haijun Jiao*,†,∥ †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing 101400, China § University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China ∥ Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany ‡

S Supporting Information *

ABSTRACT: Water adsorption and dissociation on the Fe(100) surface at different coverages have been calculated using density functional theory methods and ab initio thermodynamics. For the adsorption of (H2O)n clusters on the (3 × 4) Fe(100) surface, the adsorption energy is contributed by direct H2O−Fe interaction and hydrogen bonding. For n = 1−3, direct H2O−Fe interaction is dominant, and hydrogen bonding becomes more important for n = 4−5. For n = 6−8 and 12, structurally different adsorption configurations have very close energies. Monomeric H2O dissociation is more favored on the clean Fe(100) surface than that on H2O or OH precovered surfaces. Oassisted H2O dissociation is favorable kinetically (O + H2O = 2OH), and further OH dissociation is roughly thermo-neutral. With the increase of surface O coverage (nO, n = 2−7), further H2O dissociation has similar potential energy surfaces, and H2 formation from surface adsorbed H atoms becomes easy, while the desorption energy is close to zero for n = 7. The calculated thermal desorption temperatures of H2O and H2 on clean surface agree well with the available experiment data. The characteristic desorption temperatures of H2O and H2 coincided at 310 K are controlled by the kinetics of disproportionation (2OH → O + H2O) and dissociation (2OH → 2O + H2) of surface OH groups. The dispersion corrections (PBE-D2) overestimate slightly the adsorption energies and temperatures of H2O and H2 on iron surface. At 0.5 ML coverage (6 × OH), the adsorbed OH groups at the bridge sites do not share surface iron atoms and form two well-ordered parallel lines, and each OH group acting as donor and acceptor forms hydrogen bonding with the adjacent OH groups, in agreement with the experimentally observed surface structures. At 1 ML coverage of OH (12 × OH) and O (12 × O), the adsorbed OH groups at the bridge sites share surface iron atoms and form four well-ordered parallel lines; and the adsorbed O atoms are located at the hollow sites. Energetic analysis reveals that 1 ML OH coverage is accessible both kinetically and thermodynamically, while the formation of 1 ML O coverage is hindered kinetically since the OH dissociation barrier increases strongly with the increase of O pro-covered coverage. All these results provide insights into water-involved reactions catalyzed by iron and broaden our fundamental understanding into water interaction with metal surfaces.

1. INTRODUCTION Water chemistry on solid surfaces is of central importance to a broad range of scientific and technological processes, such as corrosion, electrochemistry, nanoparticle self-assembly, environmental chemistry, lubricants, H2O splitting, and heterogeneous catalysis.1 An atomic level investigation into water−metal interaction is essential for understanding the mechanisms of many heterogeneous catalytic reactions.2−4 Water adsorption on metal surfaces depends largely on surface structures and water−water hydrogen bonding. As pointed by Hodgson et al.,5 the first layer water adsorption on metal surfaces can be divided into three modes: (i) nonwetting adsorption (forming threedimensional (3D) clusters and bare metal); (ii) wetting adsorption (forming two-dimensional (2D) monolayer); and © 2014 American Chemical Society

(iii) partial or complete dissociation (forming surface OH and/ or O), depending on the behavior of the first layer water and adsorption temperature. On nonwetting surfaces, the weak water−metal interaction leads to the formation of 3D multilayer water clusters at low temperature, and the cluster sizes depend on adsorption temperature.6 On the more reactive close-packed surfaces, water clusters can form an intact wetting layer, while water dissociation will occur at high temperature.7−9 On open surfaces of most metals, water dissociation occurs.2,3 Received: August 12, 2014 Revised: October 17, 2014 Published: October 20, 2014 26139

dx.doi.org/10.1021/jp5081675 | J. Phys. Chem. C 2014, 118, 26139−26154

The Journal of Physical Chemistry C

Article

Table 1. Dissociation Barrier (Ea, eV), Dissociation Energy (Er, eV), and Dissociating O−H Distance (dO−H, Å) in Transition State for the Relevant Reactions of H2O on the Fe(100) Surface with Different Methods, Different Surface Sizes, and Slab Thicknesses (mLnR, m for the Total Slab Layers and n for the Relaxed Surface Layers) as well as O, H, OH, and H2O at Top (t), Bridge (b) and Hollow (h) Sites dissociation pathway H2O(t) → OH(b) +H(h) H2O(b) → OH(b) + H(b) H2O(h) → OH(t) + H(t) H2O(b) → OH(b) + H(b) H2O(b) → OH(b) + H(h) H2O(t) → OH(b) + H(b) H2O(t) → OH(b) + H(h) H2O(t) → OH(b) + H(h) H2O(t) → OH(b) + H(h)

OH(b) OH(h) OH(h) OH(b) OH(b) OH(b) OH(b)

+ H(h) → O(h) + 2H(h) → O(h) + H(h) → O(t) + H(t) → O(h) + H(h) → O(h) + H(h) → O(h) + H(h) → O(h) + H(h)

OH(b) →O(h) + H(h)

O(h) + H2O(t) → O(h) + OH(b) + H(h) O(h) + H2O(t) → 2OH(b) O(t) + H2O(t) → 2OH(t) O(h) + H2O(t) → 2OH(b) 2H2O(t) → H2O(t) +OH(b) + H(h) H2O(t) + OH(b) + H(h) → H2O(t) + O(h) + 2H(h) H2O(t) + OH(b) + H(h) → 2OH(b) + 2H(h) 2H2O(t) → H2O(t) + OH(b) + H(h) a

Size (mLnR)/method H2O → OH + H 4 × 4 (4L2R)/PBE 4 × 4 (4L2R)/PBE Fe5 cluster/ASED 2 × 2 (5L3R)/USPP-GGA 2 × 2 (5L2R)/PW91 3 × 3 (5L3R)/PBE 2 × 2 (4L1R)/PW91 2 × 2 (5L2R)/PW91 3 × 3 (5L2R)/PW91 3 × 3 (6L6R)/PW91 3 × 3 (6L6R)/HSE06 OH → O + H 4 × 4 (4L2R)/PBE 4 × 4 (4L2R)/PBE Fe5 cluster/ASED 2 × 2 (5L2R)/PW91 3 × 3 (5L3R)/PBE 2 × 2 (4L1R)PW91 2 × 2 (5L2R)/PW91 3 × 3 (5L2R)/PW91 3 × 3 (6L6R)/PW91 3 × 3 (6L6R)/HSE06 O + H2O → 2OH 4 × 4 (4L2R)/PBE 4 × 4 (4L2R)/PBE Fe5 cluster/ASED 2 × 2 (4L1R)/PW91 2H2O dissociation 4 × 4 (4L2R)/PBE 4 × 4 (4L2R)/PBE 4 × 4 (4L2R)/PBE 3 × 3 (5L2R)/PW91

Ea

Er

dO−H (TS)

ref

0.41 0.28 0.41 ∼0 0.08 0.35 0.36 1.04 0.94 0.32 0.68

−0.88 −1.04

1.328 1.262 1.38

a a 32 35 37 38 39 40

0.85 0.88 0.64 0.81 0.79 0.81 0.84 0.75 0.39 0.91

−1.1 −0.82 −0.84 −0.76 −0.77

1.28

41

−0.47 −0.65

−0.51 −0.35 −0.48 −0.64

1.432 1.281 1.33

1.40

a a 32 37 38 39 40 41

0.90 0.18 0.15 0.18

−0.74 −0.56

0.49 1.51 0.93 1.25

−0.65

1.423 1.057/1.469 1.38 1.13/1.31

a a 32 39

−1.03 −0.17 −0.73

1.333 1.422 1.446

a a a 40

This work.

molecules via hydrogen bonding. A recent density functional theory (DFT) study of water adsorption on the Ru(0001) surface favors a planar geometry having all six water molecules bonding to Ru,27 where some water molecules in the overlayer are dissociated, yielding a partially dissociated phase. Water-induced oxidation on iron surfaces has been the subject of enormous interest for many years. Especially, the low index iron surfaces have been used as a prototypical substrate for studying the interaction of H2O and Fe surface.31−41 On the basis of low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), Dwyer et al.,31 found that molecular water adsorption on the Fe(001) surface forms a disordered c(2 × 2) structure and proposed that the formed atomic oxygen or hydroxyl group occupy the hollow sites. On the basis of high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) studies, Hung et al.33 concluded that molecular water adsorbs at 100 K on the Fe(100) surface and desorbs from three states at 165, 220, and 310 K. They suggested that the hydrogen-bonded water clusters are formed sequentially on the surface at low exposure, and 165 K is the desorption temperature of an icelike multilayer/cluster. Water molecules interacting directly

Due to its simplicity and importance, the adsorption of mono molecular water on many metal surfaces has been studied, such as on the Cu,4,10−13 Co,14−16 Ni,4,10,17,18 Ru,10,19 Rh,10,17,18 Pd,4,10,18 Ag,10,18 Pt,4,20 and Au4,10 surfaces. These studies found that molecular water adsorbs at atop of the surface metal atom with the two O−H bonds nearly parallel to the surface. Moreover, water dimer, trimer, hexamers and the growth of 2D ice were recently studied on metal surfaces, such as on Pt(111), 21−25 Ru(0001), 26,27 Rh(111), 21 Pd(111), 21 Au(111),21 Ag(111),28 Cu(111),13,28 and Cu(110).7−9,29,30 As water coverage increases hydrogen bonding networks of various phases are formed and they continue to grow into multilayers and ice structures. Water oligomers usually form buckling structures, which are the results of the competition between water−metal interaction and water−water hydrogen bonding. At low temperature, water hexamer represents the basic structure unit, which is to make up an ice water bilayer and usually observed on many metal surfaces. In the buckling structures of water hexamer, each water molecule acts as a single donor and single acceptor, and the first layer water molecules bond directly with surface metal atoms, while the second layer water molecules interact with the first layer water 26140



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saddle point). The lattice parameter is optimized using the body-centered cubic (bcc) unit cell, and its reciprocal space is sampled with 15 × 15 × 15 k-point grid generated automatically by using the Monkhorst−Pack method.50 The optimized lattice constant of 2.835 Å is close to the experimental value of 2.866 Å,51 and the calculated magnetic moment (2.23 μB) is also close to the experimental value (2.22 μB).52 (b). Model. Although we are very confident in modeling different Fe surfaces,53−62 we have tested the effects of the number of the relaxation layers, k-points, and surface size in H2O adsorption energy to find reasonable slab models for H2O adsorption.35,37−40 All these results are listed in the Supporting Information. For the Fe(100) surface, a periodic slab with a vacuum region in 15.0 Å width was used to separate the periodically repeated slabs. The surface structural relaxation and the total energy calculation were performed with the 3 × 3 × 1 Monkhorst−Pack k-point sampling. A (3 × 4) surface size was used. We used a four-layer model, where the top two layers including adsorbates were relaxed, while the bottom two layers were fixed in their bulk positions. The structures include totally 48 iron atoms. The top and side reviews along with the possible adsorption sites of the Fe(100) surface are shown in Figure 1. Water clusters in the gas phase were calculated in a (15 × 15 × 15) cell.

with surface iron atoms desorb at 220 K. As the temperature increases to 243 K, molecular water disappears completely, and a layer of hydroxyl groups tilted on the surface at the bridge site is observed. At temperatures up to 310 K, the disproportionation of the adjacent OH species, forming molecule H2O and atomic oxygen (2OH → O + H2O), is observed; and at the same time, the decomposition of OH into H and O atoms is possible (2OH → 2O + H2), as characterized by H2 desorption. Theoretically, there are many reports about water chemistry on the Fe(100) surface. Many studies have confirmed that molecular water adsorbs atop of the surface Fe atom with the two O−H bonds nearly parallel to the surface. Eder et al.35 found that water on all most symmetric sites has similar adsorption energies, and water adsorption on the bridge site with an upright configuration is slightly more stable, leading to spontaneous water dissociation into surface OH and H species, which are on the bridge sites. However, this barrier-less dissociation is contrary to the conclusion that the molecular adsorbed species is stable at 100 K as found by Hung et al.33 As listed in Table 1, the reported water dissociation barriers differ strongly from the models and methods, and therefore a direct comparison is not straightforward. Apart from the energetic difference about water dissociation, the detailed analysis about water aggregation has not been reported. To gain systematic insight into the chemistry of water on the Fe(100) surface, we have calculated the adsorption structures and energies for all species involved in water dissociation. The key issues of our study are the preferred adsorption sites; coverage dependent adsorption structures; factors determining the adsorption energies; surface oxygen mediated water dissociation; as well as desorption temperatures of water and hydrogen molecules on the surface.

2. COMPUTATIONAL DETAILS (a). Method. All calculations were done with the plane wave-based pseudopotential code in Vienna ab initio simulation package (VASP).42,43 The electron−ion interaction is described with the projector augmented wave (PAW)44,45 method. The exchange and correlation energies are described using the spinpolarized generalized gradient approximation and Perdew− Burke−Ernzerhof functional (GGA-PBE).46 It is important to mention that the GGA is crucial for achieving a correct description of the structural and magnetic ground states of bulk Fe47 and necessary for the accurate treatment of water structures and hydrogen bonding.21 Due to the weak adsorption of water on the metal surface, we have carried out long-range dispersion correction for van der Waals (vdW) interactions on the basis of the semiempirical GGA-type functional (PBE-D2) proposed by Grimme.48 To ensure sufficiently accurate energies with errors due to smearing of less than 1 meV per unit cell, a cutoff energy of 400 eV and the Gaussian electron smearing method with σ = 0.20 eV are used. The geometry optimization is done when the forces acting on the atoms were smaller than 0.02 eV/Å. As an energy convergence criterion, the optimization is done when the maximum difference between the output and the input of each element of the density matrix in a self-consistent field cycle is smaller than 10−4. Spin-polarized calculations are performed to account for the magnetic properties of iron. Transition state structures were estimated using the climbing image nudged elastic band method (CI-NEB).49 For each optimized stationary point vibrational analysis was performed at the same level of theory to determine its nature (either minimum or

Figure 1. Top (a) and side (b) views of the surface structures of Fe(100) as well as possible adsorption sites: on-top (t), bridge (b), 4fold-hollow (h).

The adsorption energy was calculated by using the expression as usually defined in eq 1, where EX/slab is the total energy of the slab with adsorbed molecules in its equilibrium geometry, Eslab is the total energy of the clean surface, and EX is the energy of the free adsorbate in gas phase. Therefore, the more negative the Eads, the stronger the adsorption. It is noted that the reported adsorption energies do not include the corrections of zero-point energies (Eads), since they have little effect on the surface reaction and mainly affect the gas molecules.63 Eads = E X/slab − Eslab − E X

(1)

The dissociation barrier (Ea) and reaction energy (Er) are calculated according to eqs 2 and 3, where EIS, ETS, and EFS are the energies of the corresponding initial state (IS), transition state (TS), and the final state (FS), respectively. Ea = E TS − E IS

(2)

Er = E FS − E IS

(3)

For studying the factors governing the stability of the surface oligomers, we defined the stepwise hydrogen bonding energy 26141



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Figure 2. Adsorption configurations and adsorption energies (eV) of H, O, OH, and H2O at the high-symmetry adsorption sites on Fe(100) surface (surface/blue; oxygen/red; hydrogen/yellow).

vibration to the solid surfaces is negligible. Thus, we use the DFT calculated total energy to substitute the Gibbs free energy of the solid surfaces and the Ggas(H2O) equals μ(H2O). The chemical potential of H2O (μ(H2O)) can be described as

(EH‑bond/ads/gas) of the latest (nth) H2O molecule in a water oligomer (H2O)n in eq 4, where E(H2O)n/ads/gas, E(H2O)n‑1/ads/gas and E(H2O)/nth/ads/gas are the gas phase single-point energies on the geometries taking directly from the adsorbed (H2O)n/ads, (H2O)n‑1/ads, and (H2O)nth/ads on the surface.

μH O(T , P) = E Htotal + μH ̃ O (T , P 0) + kBT ln(PH2O/P 0) 2O

E H − bond/ads/gas = E(H2O)n /ads/gas − E(H2O)n−1 /ads/gas − E(H2O)nth /ads/gas

2

EHtotal states for the DFT calculated energy of the isolated 2O H2O gas phase molecule (including zero point vibrations), and μ̃H2O(T, P0) is the chemical potential at different temperatures, which can be found in thermodynamic tables. The last term kBT ln(PH2O/P0) is the contribution of the temperature and H2O partial pressure to the chemical potential. Therefore, eq 6 can be rewritten as

(4)

In addition, we also defined the adsorption energy (EH2O/nth/ads) of the nth H2O molecule of the adsorbed (H2O)n cluster on the surface in eq 5, where EH2O/nth/slab is the total energy of the adsorbed nth H2O molecule, EH2O/nth/ads/gas is the total energy of the nth H2O molecule in gas phase, and Eslab is the total energy of the slab. All these total energies are single-point energies. E H2O/ nth/ads = E H2O/ nth/slab − E H2O/ nth/ads/gas − Eslab

2

ΔG = E[Fe(100)] − E[{H 2O}/Fe(100)] + E Htotal 2O + μH ̃ O (T , P 0) + kBT ln(PH2O/P 0)

(5)

2

(c). Atomistic Thermodynamics. Atomistic thermodynamics64,65 is a convenient tool to solve problems referring to real reaction conditions.66−70 According to the literature, we take H2O desorption on Fe(100) surface, H2O/Fe → Fe + H2O(g), as an example; and the change of Gibbs free energy (ΔG) for this reaction can be described as eq 6.

(7)

3. RESULTS AND DISCUSSIONS 3.1. Adsorption of O, H, OH, and H2O. The adsorption of H, O, OH, and H2O on the Fe(100) surface at the 4-fold hollow (h), bridge (b), and top (t) sites by considering different initial configurations (Figure 1) was computed. All adsorption configurations and their structural properties are listed in Figure 2 and Table 2. For one H atom, the adsorption configuration at the b and h sites has close adsorption energies (−0.45 and −0.42 eV, respectively), while that at the t site is much less stable (−0.06 eV). These results agree with the previous studies,35,38−40 although it is also reported that

ΔG = G[Fe(100)] + Ggas(H 2O) − G[{H 2O}/Fe(100)] (6)

In this equation, G[Fe(100)] is the Gibbs free energy of the clean Fe(100) surface, while G[{H2O}/Fe(100)] is the Gibbs free energy of the Fe surface with adsorbed H2O molecule. Because of the large mass differences, the contribution of 26142



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nearly parallel to the iron surface (−0.41 eV), while the adsorption configurations at the b and h sites are much less stable (−0.27 and −0.07 eV, respectively). All our results are in agreement with the previous studies.35,38−40 3.2. Structures of (H2O)n in the Gas Phase. There are many studies about the structures and energies of water clusters and oligomers in the gas phase at various levels of theory.71−78 In our calculation, the gas phase structures of water clusters are free and fully optimized without any constrains. The optimized most stable water clusters of (H2O)n (n = 2−6) in the gas phase are shown in Figure 3. The computed structural parameters and energetic data are listed in Table 3 along with some available literature results for comparison.

Table 2. Computed Adsorption Energies (Eads, eV), Distances (d, Å) between Surface Fe Atom to the Adsorbed O, H, OH, and H2O as well as the O−H Distance and HOH Angle θHOH (deg) in the Optimized Water h site Eads

dH−Fe Eads

dO−Fe Eads

dO−Fe dO−H

−0.42 (−2.68a) −0.3535 −0.4238 (−2.69)39 −0.4240 (−2.7040a) 1.76 −3.43 (−6.84b) −3.7235 −3.4738 (−6.5939b) −3.9540 (−6.8540b) 2.05 −3.98d −3.8635 −3.8839 −3.8440 2.22d 0.982d 1.0135 0.9839

Eads

dO−Fe dO−H

−0.07c (−0.03d)

−0.0939d −0.3740c (−0.2540d) 3.66c; 3.38d 0.976c (0.975d)

39d

ϑHOH

0.97 103.5c (105.0d)

105.1

39d

b site H −0.45 (−2.71a) −0.3635

t site −0.06 (−2.21a) 0.1735

−2.7139 −0.3740 (−2.6440a) 1.71 O −2.80 (−6.21b) −3.1635

−2.6939 −0.2440 (−2.0340a) 1.93

(−6.0939b) −3.3040 (−6.2040b) 1.82 OH −4.14c (−4.04d) −4.1235 −3.9538 −4.2239c (−4.1239d) −4.0140 1.99c (1.97d) 0.976c (0.971d) 1.0135 0.9838 0.9839c (0.9739d) H2O −0.27d −0.3535 −0.2638

(−5.2839b) −2.6240 (−5.5540b) 1.64

−0.3439d −0.3640c (−0.2640d) 2.40d 0.980d 1.0135 0.9838 0.9839d 108.6d 11135 108.538 109.339d

−0.46;39 0.9839 −0.3840c (−0.0440d) 2.22c 0.985c

−2.09 (−5.50b) −2.1935

−3.70c (−3.58d) −3.7135

Figure 3. Top (a) and side (b) views of the gas phase structures of (H2O)n clusters.

−3.7339c (−3.6239d) −3.5440 1.82c (1.80d) 0.974c (0.967d) 0.9135

For the most stable water clusters of (H2O)n (n = 2−5) in the gas phase, the optimized structures agree with the available literature results. For example, (H2O)2 has a chain structure containing one hydrogen bonding interaction, and the (H2O)n (n = 3−5) structures are monocyclic and have three, four, and five hydrogen-bonding interactions, respectively. Instead of the cyclic, prism, or cage structures, we have computed a (H2O)6 structure containing two fused four-membered rings on the basis of the stable adsorbed structure of (H2O)6 on the Fe(100) surface, although it is not the most stable one in gas phase. The most important parameters for these water clusters are their aggregation energies and hydrogen bonding distances. For the most stable clusters ((H2O)n, n = 2−5; Table 3), the aggregation energy increases and the distances of the hydrogen bonding decreases along with the increase of the cluster size. It is noted that the aggregation energy per hydrogen bonding increases with the cluster sizes. The nonbonded dO−O distance decreases with the cluster size increase. This is the same trend as reported previously.77 As reported in the literature78 the aggregation energy of the most stable clusters depends strongly on methods and basis sets. As shown in Table 3, our computed aggregation energies for (H2O)n (n = 2−5) are in reasonable agreement with the results obtained at B3LYP and MP2 levels of theory; while our (H2O)6 structure is much less stable than the prism isomer with nine hydrogen bonding interactions. 3.3. Growth of (H2O)n on Fe(100). Now we consider the growth of water clusters on the Fe(100) surface. Due to the adsorption of water on iron surface and hydrogen bonding interaction among adsorbed water molecules, we used sequential (or stepwise) adsorption for finding the most stable adsorbed water clusters on the iron surface. For example, one additional water molecule was added to the previous most stable one for getting the next most stable one after considering both interactions. The structures of the most stable adsorption configurations are given in Figure 4, and the computed adsorption energies and bond parameters are listed in Table 4.

0.9739c (0.9739d) −0.41c −0.3938

0.9838 104.5c 105.138 105.139

a

Referencing to hydrogen atom in parentheses. bReferencing to oxygen atom in parentheses. cFor titled adsorption configuration to the surface. dFor perpendicular configuration to the surface in parentheses.

hydrogen atom adsorption at the b site is more stable than that at the h site. For one O atom, the adsorption configuration at the h site (−3.43 eV) is much more stable than those at the b and t sites (−2.80 and −2.09 eV, respectively). For one OH group, the most stable adsorption configuration with the O−H bond tilted from the surface normal is at the b site (−4.14 eV), whereas those at the h site in a vertical orientation and at the t site in a tilted configuration are less stable (−3.98 and −3.70 eV, respectively). For one H2O molecule, the most stable adsorption configuration is at the t site with H2O molecule 26143



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Table 3. Aggregation Energy (Eagg, eV), Average Hydrogen Bonding Energy (EH‑Bond, eV), Nonbonded O−O Distances (dO−O, Å), Angle of the Three Nearest O Atoms (ϑo‑o‑o, degree), the Distance of the Hydrogen Bonding (dH‑bond, Å) of Gas Phase (H2O)n Eagg

EH‑Bond dO−O ϑo‑o‑o dH‑bond

(H2O)2

(H2O)3

(H2O)4

(H2O)5

(H2O)6

−0.24 (−0.18)a (−0.21)b (−0.23)c −0.24 2.881

−0.76 (−0.52)a (−0.67)b (−0.72)c −0.25 2.718; 2.764; 2.774 58.8; 60.4; 60.8 1.782; 1.866; 1.858

−1.24 (−0.91)a (−1.19)b (−1.25)c −0.31 2.709; 2.709; 2.710; 2.709 89.7; 89.7; 90.3; 90.3 1.725; 1.723; 1.724; 1.722

−1.78 (−1.21)a (−1.57)b (−1.64)c −0.36 2.662; 2.668; 2.669; 2.690; 2.685 107.0; 107.1; 107.5; 107.7; 107.7 1.651; 1.659; 1.661; 1.685; 1.681

−1.37 (−1.48)a,d (−1.92)b,d (−2.11)c,d −0.23 2.879; 2.885; 2.902; 2.923; 2.935; 3.000; 3.003 87.1; 88.7; 91.6;92.4; 87.9; 88.6; 91.4; 92.1 1.910; 1.915; 1.937; 1.958; 1.998; 2.044; 2.046

1.901

a

At HF/6-311++G(2d,2p). bAt B3LYP/6-311++G(2d,2p). cAt MP2/6-311++G(2d,2p). dFor the prism structure containing nine hydrogenbonding interactions. All these are taken from ref 78.

Figure 4. Most stable structures and the adsorption energy (eV) of water monomer and small clusters adsorbed on the Fe(100) surface (surface/ blue; oxygen/red; hydrogen/yellow).

For the adsorbed (H2O)2, both H2O molecules adsorb stably at the top sites. Taking the Fe−O distance of one H2O adsorption as reference (2.215 Å), one H2O binds more tightly with the surface Fe atom (2.158 Å), while the second one binds less tightly with the surface Fe atom (2.444 Å). In addition, the hydrogen bonding and nonbounded O−O distances (1.838 and 2.802 Å, respectively) are shorter than those in gas phase (1.901 and 2.881 Å, respectively). The adsorption energy is −0.93 eV, which is higher than twice the H2O adsorption (−0.82 eV), indicating the hydrogen bonding contribution. The computed stepwise hydrogen bonding energy is −0.19 eV. The

adsorption energy of the second adsorbed H2O molecule is −0.34 eV, indicating that the second H2O adsorption is dominant. For the adsorbed (H2O)3, all three H2O molecules adsorb stably at the top sites and form two hydrogen-bonds. In contrast to the triangular structure with the O−O−O angles close to 60° in gas phase, the three adsorbed H2O molecules have one O−O−O angle close to 90°. Compared to the distance of one H2O adsorption (2.215 Å), one Fe−O distance is shorter (2.132 Å), while the other two Fe−O distances become longer (2.401 and 2.436 Å). In addition, the hydrogen 26144



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Table 4. Adsorption Energies (Eads, eV), Shortest Distances (dO−Fe, Å) between O Atom and Surface Fe Atom as well as the Distances (dH‑bond, Å) of Hydrogen Bonding Eads

dO−Fe

(H2O)1 (H2O)2 (H2O)3 (H2O)4 (H2O)5 (H2O)6 (H2O)7

−0.41 −0.93 −1.47 −2.10 −2.68 −3.33 −3.93

(H2O)8

−4.51

2.215 2.158; 2.444 2.132; 2.401; 2.436 2.256; 2.287; 2.293; 3.112 2.248; 2.262; 2.277; 3.200; 3.998 2.231; 2.236; 2.243; 3.177; 3.185; 3.200 2.166; 2.173; 2.297; 3.281; 3.521; 3.606; 3.612 2.166; 2.169; 2.257; 2.362; 3.282; 3.531; 3.604; 3.617

dO−O 2.802 2.833; 2.840 2.628; 2.681; 2.749; 2.945 2.552; 2.650; 2.700; 2.741; 2.789; 2.793; 2.805; 2.829; 2.607; 2.619; 2.644; 2.653; 2.907; 3.166 2.624; 2.628; 2.643; 2.654; 2.925; 2.955; 3.179

dH‑bond

2.840 2.833; 2.839; 2.843 2.659; 2.736; 2.943; 2.665; 2.732; 2.815;

1.838 1.870; 1.889 1.613; 1.737; 1.819; 2.025 1.498; 1.671; 1.692; 1.790; 1.786; 1.790; 1.804; 1.927; 1.601; 1.617; 1.645; 1.647; 1.972; 2.292 1.607; 1.640; 1.646; 1.648; 1.941; 1.985; 2.301

1.866 1.973; 1.980; 1.996 1.649; 1.771; 1.922; 1.655; 1.765; 1.847;

energy is −0.81 eV, which is slightly higher than that of the aggregation of the fourth H2O molecule in the adsorbed (H2O)4, and the adsorption energy of the fifth adsorbed H2O molecule is only −0.06 eV. Taking the adsorption of (H2O)3 as reference (−1.47 eV), the adsorption (−2.68 eV) of (H2O)5 should be determined by the direct Fe−O and the hydrogen bonding interactions. The most stable structure of the adsorbed (H2O)6 has two fused four-membered rings and is therefore bicyclic, in which three H2O molecules interact directly with the surface Fe atoms (2.231, 2.236, and 2.243 Å); these distances are shorter than those in the adsorbed (H2O)4 and (H2O)5 clusters, and three H2O molecules have longer Fe−O distances (3.177, 3.185, and 3.200 Å). The nonbonded O−O distances are in the same range (2.789, 2.793, 2.805, 2.829, 2.833, 2.839, and 2.843 Å), and the shortest nonbonded O−O distances in (H2O)6 are shorter than those in gas phase. Three of the seven hydrogen bonding distances are shorter (1.786, 1.790, 1.804, 1.927, 1.973, 1.980, and 1.996 Å), and they are the distances between the adjacent lower and higher water molecules. The adsorption energy is −3.33 eV, higher than 6-fold of single H2O adsorption (−2.46 eV). The second stable one is formed on the basis of the adsorbed (H2O)5 connecting an exocyclic H2O molecule; and the adsorption energy is −3.23 eV. Both isomers also are very close in energy. Compared to the adsorbed (H2O)5, the sixth H2O molecule adsorb at the hollow site. All the structural parameters of the first five H2O molecules in Fe−O (2.202, 2.246, 2.253, 3.225, and 3.990 Å), O−O (2.259, 2.600, 2.709, 2.714, and 2.844 Å), and hydrogen bonding distances (1.566, 1.600, 1.711, 1.752, and 1.870 Å) are in the same range of the adsorbed (H2O)5. The sixth one has longer Fe−O distance (3.718 Å) and shorter hydrogen bonding distance (1.686 Å). The computed stepwise hydrogen bonding energy is −0.18 eV, and the adsorption energy of the sixth adsorbed H2O is −0.11 eV. This indicates that the adsorption of the sixth H2O should be determined by direct adsorption and hydrogen bonding. On the basis of the structures of (H2O)6, we have computed two isomers for (H2O)7. The first one can be considered as two fused five-membered rings, where one H2O molecule interacts with two H2O molecules in the next periodic cell via hydrogen bonding. The adsorption energy is −3.93 eV. Three H2O molecules interact directly with the surface Fe atoms and the Fe−O distances (2.166, 2.173, and 2.297 Å) are shorter than those in the adsorbed (H2O)4 and (H2O)5, while four H2O molecules have longer Fe−O distances (3.281, 3.521, 3.606, and 3.612 Å). There are six shorter (1.601, 1.617, 1.645, 1.647, 1.649, and 1.771 Å) and three longer (1.922, 1.972, and 2.292 Å) hydrogen bonding distances. The second isomer can be considered as the adsorbed (H2O)5 having two exocyclic H2O

bonding and non-bounded O−O distances (1.870/1.889 and 2.833/2.840 Å, respectively) are longer than those of the gas phase (H2O)3 (Table 3) and those of the adsorbed (H2O)2 cluster (Table 4). The adsorption energy is −1.47 eV, which is higher than 3-fold of one H2O adsorption (−1.23 eV), indicating the hydrogen bonding contribution. The computed stepwise hydrogen bonding energy is −0.12 eV. The adsorption energy of the third adsorbed H2O molecule is −0.36 eV, indicating that the third H2O adsorption remains dominant. For the adsorbed (H2O)4, four H2O molecules form a cyclic structure as also found in gas phase. The first three H2O molecules at the top sites have Fe−O distances of 2.256, 2.287, and 2.293 Å, and the Fe−O distance of the last one is longer (3.112 Å). They are longer than that of singly adsorbed H2O (2.215 Å). There are one longer (2.945 Å) and three shorter (2.628, 2.681, and 2.749 Å) nonbonded O−O distances, and they are even shorter than those in the adsorbed (H2O)3 and (H2O)2 clusters. The shortest nonbonded O−O distances in (H2O)4 are also shorter than those in the gas phase (2.709 Å). Three of the four hydrogen bonding distances (1.613, 1.737, 1.819, and 2.025 Å) are shorter than those in the adsorbed (H2O)3 and (H2O)2 clusters, and the shortest one is even shorter than the shortest one in gas phase (1.722−1.725 Å). The adsorption energy is −2.10 eV, higher than 4-fold of one H2O adsorption (−1.64 eV), and this indicates the hydrogen bonding contribution. The computed stepwise hydrogen bonding energy is −0.71 eV, and the adsorption energy of the fourth adsorbed H2O molecule is −0.12 eV. This reveals that the hydrogen bonding rather than the direct interaction dominates the aggregation of the fourth H2O molecule in the adsorbed (H2O)4 cluster. For the adsorbed (H2O)5, the five H2O molecules form a cyclic structure on the basis of the adsorbed (H2O)4 structure, where four H2O molecules adsorb at the top sites and the last one is at the hollow site. The first three H2O molecules have shorter Fe−O distances (2.248, 2.262, and 2.277 Å), while those of the last two molecules (3.200 and 3.998 Å) are longer. There are one longer (2.840 Å) and four shorter (2.552, 2.650, 2.700, and 2.741 Å) nonbonded O−O distances. The shortest nonbonded O−O distances in (H2O)5 is shorter than those in gas phase (2.662 Å). The five hydrogen bonding distances (1.498, 1.671, 1.692, 1.790, and 1.866 Å) are shorter than those in the adsorbed (H2O)3 and (H2O)2 clusters and the shortest one is shorter than the shortest one in gas phase (1.651−1.685 Å). The adsorption energy is −2.68 eV, higher than 5-fold of single H2O adsorption (−2.05 eV). Since two H2O molecules have very long Fe−O distances, it is to expect that hydrogen bonding dominates the aggregation of the last two H2O molecules. Indeed, the computed stepwise hydrogen bonding 26145



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Figure 5. Optimized geometries and the adsorption energy (eV) for the stationary points in the reaction of H2O direct dissociation on Fe(100) surface (surface/blue; oxygen/red; hydrogen/yellow).

molecules, and the adsorption energy is −3.91 eV. Both isomers also are very close in energy. The first six H2O molecules have Fe−O (2.156, 2.224, 2.231, 3.202, 3.575, and 3.995 Å) and O−O (2.621, 2.637, 2.655, 2.657, 2.722, and 2.834 Å) as well as hydrogen bonding distances (1.594, 1.640, 1.656, 1.683, 1.720, and 1.859 Å) in the same range of the adsorbed second stable (H2O)6. The seventh one has longer Fe−O distance (4.072 Å) and forms two hydrogen binding (1.805 and 1.867 Å). The computed stepwise hydrogen bonding energy is −0.39 eV, and the adsorption energy of the seventh adsorbed H2O is −0.14 eV. This indicates that the adsorption of the seventh H2O should be determined dominantly by hydrogen bonding. For the adsorbed (H2O)8, we have computed three structurally very different isomers, however, they have very close adsorption energies (−4.51, −4.34, and −4.41 eV). One is formed on the basis of the adsorbed (H2O)7-t1 connecting an exocyclic H2O molecule, and the structural parameters are close to those of the adsorbed (H2O)7-t1, while the eighth H2O has Fe−O distance of 2.362 Å and hydrogen bonding distance of 1.847 Å. The computed stepwise hydrogen bonding energy is −0.13 eV, and the adsorption energy of the eighth adsorbed H2O is −0.38 eV, indicating that the eighth H2O adsorption is dominant. The (H2O)8-t2 is formed on the basis of the structures of (H2O)5, like (H2O)7-t1, and also can be considered as two fused five-membered rings, where two H2O molecules connect the two rings. The (H2O)8-t3 is the same as the adsorbed (H2O)6-t1, in which four H2O molecules interact directly with the surface Fe atoms at the same height (2.241 Å), and four molecules have longer (3.201 Å) Fe−O distances at the same height. It is therefore to conclude that for (H2O)n clusters on the Fe(100) surface, direct Fe−O interaction is dominant for n = 1−3, while the hydrogen bonding becomes more important for n = 4−5. For n = 6−8, structurally different adsorption configurations are very close in energies. In addition, it is to note that under the circumstance of limited water amount, the stable configuration of adsorbed water clusters is a pentagonbased structure by sequential addition; otherwise, the preferred configuration is based either on four-membered rings or on five-membered rings, which are very close in energy. 3.4. H2O Dissociation on Clean Surface. (a). H2O Direct Dissociation (H2O → O + H2(g)). On the basis of the most stable H2O adsorption configuration, we computed the dissociation pathways into surface H, OH, and O. The optimized geometries for the stationary points are shown in Figure 5, their structure parameters are shown in Table 5. The reaction barriers, the reaction energies and the structure parameters of the transition states are given in the Supporting

Table 5. Adsorption Energies Eads (eV) and Bond Distances (d, Å) of the IS and FS for H2O Direct Dissociation and 2H2O Dissociation on the Fe(100) Surface Eads H2O(t) OH(b) + H(h) O(h)+2H(h)

H2O(t) + H2O(t) 1H2O(t) + OH(b) + H(h) 1H2O(t) + O(h) + 2H(h) 2OH(b) + 2H(h)

dFe−O

dFe−H

H2O direct dissociation 2.215 1.983; 1.984 1.705; 1.705 2.047; 2.053; 2.057; 1.874; 1.880; 2.378; 2.211; 2.113 1.861; 1.972; 2.164; 1.918 2H2O dissociation −0.93 2.158; 2.444

−0.41 −1.29 −1.76

−1.96

2.117; 2.055; 2.066

1.924; 1.937; 2.153; 2.054

−2.13

2.224; 2.031; 2.243; 2.037; 2.060 1.965; 1.972; 2.016; 2.028

1.842; 2.365; 2.514; 1.919; 1.834; 2.047; 2.310; 2.062 1.832; 2.486; 2.513; 1.873; 1.874; 1.890; 2.321; 2.194

−2.69

Information. The total potential energy surfaces are shown in Figure 6. In order to compare with the previous studies, we also considered H2O dissociation at the bridge site and OH dissociation at the hollow site. The optimized geometries and structural parameters for the stationary points are listed in the Supporting Information. Although it is not straightforward, we tried to compare our results with those from the literature. For H2O dissociation at top site, the activation barrier is 0.41 eV, and the reaction with the formed OH at the bridge site and H at the hollow site is exothermic by 0.88 eV. In the transition state TS1(t), the breaking O−H distance is 1.328 Å. For H2O dissociation at the bridge site, the activation barrier is 0.28 eV, and the reaction with the formed OH and H at bridge sites is exothermic by 1.04 eV. In the transition state TS1(b), the breaking O−H distance is 1.262 Å. The lower barrier and stronger exothermic property of H2O dissociation at the bridge site than at the top site are due to their different H2O adsorption energy at bridge and top sites (−0.27 vs −0.41 eV) as well as the H adsorption energy at bridge and hollow sites (−0.45 vs −0.42 eV; Table 2). Considering the energy difference (0.14 eV) in adsorption at top and bridge sites, both transition states are close in energy, indicating that both dissociation paths are favorable. Our computed H2O dissociation barrier (0.41 eV) at the most stable adsorbed top site agrees reasonably with the results on smaller surface sizes using the same method (0.35 eV38) and using PW91 (0.36 eV39 and 0.32 eV41) or even using a Fe5 cluster model (0.41 eV32) but differs strongly from those reported by Freitas et al. using PW91 (1.04 and 0.94 eV).40 Since not all parameters of the reported transition state 26146



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Figure 6. Potential energy surfaces (in eV) for the dissociation reaction of H2O and 2H2O on the Fe(100) surface. The red data in the parentheses are the relevant reaction barriers (s for surface species; and g for gaseous species).

2H2O → OH + H + H2O and (ii) OH + H + H2O → O + 2H + H2O. The optimized geometries for the stationary points in the 2H2O dissociation reaction are shown in the Supporting Information, and the structure parameters of these points are shown in Table 5. The reaction barriers, the reaction energies, and the structure parameters of the transition state are given in the Supporting Information. The total potential energy surfaces are shown in Figure 6. In the first step, the dissociation barrier is 0.49 eV, slightly higher than that (0.41 eV) of one H2O dissociation; but much lower than that (1.25 eV) reported by using PW91 and a smaller supercell (3 × 3).40 This difference is also caused by the different transition state structures. In the transition state TS3, the breaking O−H distance is 1.333 Å. In addition, the dissociation is exothermic by 1.03 eV, stronger than that (0.88 eV) of one H2O dissociation. In the coadsorbed OH + H + H2O state, the hydrogen bonding distance between the adsorbed H2O and OH is 1.613 Å, shorter than that (1.838 Å) in the coadsorbed (H2O)2 systems, implying stronger interaction between the coadsorbed H2O and OH on the surface. In the second step, the dissociation barrier via TS4 is 1.51 eV, much higher than that (0.85 eV) of the coadsorbed OH + H; and the dissociation is slightly exothermic by 0.17 eV. In addition, we also computed the OH + H + H2O → 2OH + 2H; and the dissociation barrier via TS5 is 0.93 eV, and the dissociation is exothermic by 0.73 eV. Considering the higher barriers of H2O dissociation mediated by both coadsorbed H2O and OH, we paid our attention to the stepwise H2O adsorption and dissociation on O-precovered surface after the formation of gaseous H2 as discussed below. 3.5. H2O Dissociation on O Precovered Surface. On O precovered surface, H2O dissociation is modeled with the stepwise increase of H2O molecules after the formation of gaseous H2, i.e., nO + H2O(g) → (n+1)O + H2(g) (n = 1−7). The optimized structures of IS, TS, and FS; the computed bond lengths of IS and FS are given in the Supporting Information. The dissociation barriers, dissociation energies,

structures are available, it is not easy to understand such differences. Despite such differences in dissociation barriers, the dissociation energies from different models and methods are in very close range. For H2O dissociation at the less stable bridge site, our computed dissociation barrier (0.28 eV) differs also from the reported result by Eder et al. (∼0 eV at USPP-GGA35) and Lo et al. (0.08 eV at PW9137). Such barrier-less dissociation of water does not agree with the experiment, which shows that water can adsorb molecularly at 100 K. Nevertheless, our computed dissociation energy (−1.04 eV) agrees with the reported data (−1.1 eV at USPP-GGA35). These differences in dissociation barriers are the results of different ways in searching the transition states. Such unbelievable differences have been indeed observed for formic acid dissociation.79 For OH dissociation, we considered the coadsorption of OH(b) and H(h). The computed dissociation barrier is 0.85 eV and the dissociation into (O(h) + 2H(h)) is exothermic by 0.47 eV, in agreement with most of the available data (Table 1). In the transition state TS2(b), the breaking O−H distance is 1.432 Å. In addition, we also considered OH dissociation at the hollow site OH(h), the computed activation barrier is 0.88 eV and the dissociation into (O(h)+H(h)) is exothermic by 0.65 eV. In the transition state TS2(h), the breaking O−H distance is 1.281 Å. Considering the energy difference (0.16 eV) in adsorption at the bridge and hollow sites, OH dissociation at the bridge should be more favorable kinetically. As shown in Figure 6, the most stable state of H2O dissociative adsorption is the coadsorbed O + 2H; and the dissociation is exothermic by 1.76 eV. The formation of gaseous H2 from coadsorbed O + 2H is endothermic by 0.86 eV. However, the formation of O + H2(g) is exothermic by 0.49 to the adsorbed H2O or by 0.90 eV to gaseous H2O. (b). H2O Dissociation at Higher Coverage (2H2O = O + H2(g) + H2O). For the case with the increased coverage, the model of two coadsorbed H2O molecules interacting directly with the surface iron atoms is used. For the two coadsorbed H2O molecules, there are two steps for H2O dissociation: (i) 26147



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Figure 7. Potential energy surfaces (in eV) for H2O dissociation on nO (n = 2−7) precovered Fe(100) surface. The red data in the parentheses are the relevant reaction barriers (s for surface species; and g for gaseous species).

exothermic by 0.56 eV. The reverse reaction, or the disproportionation (2OH = O + H2O) reaction, needs an energy barrier of 0.74 eV and is endothermic by 0.56 eV. In contrast, direct dissociation of the coadsorbed O + H2O into the coadsorbed O + H + OH needs an energy barrier of 0.90 eV, which is kinetically unfavorable. This agrees with the previously reported results that O precovered surface can mediate H2O dissociation very easily into OH + OH than into O + H + OH.10,20,39,63 For the next dissociation step (2OH = O + H + OH), the activation barrier is 0.97 eV and the dissociation energy is only −0.02 eV. In the transition state (TS7), the O−H distance is 1.411 Å. The formed O and H atoms move to the hollow sites. Prior to the further dissociation of the second OH, the OH can

and critical bond distances of TS are given in the Supporting Information. The energy profiles are shown in Figure 6 for n = 1 as well as in Figure 7 for n = 2−7. (a). H2O Dissociation on One O Precovered Surface (O + H2O(g) → 2O + H2(g)). The model begins with the coadsorption of the previously formed surface O and one gas phase H2O. The adsorption energy of H2O is −0.45 eV, which is slightly higher than that (−0.41 eV) on the clean surface. In the O + H2O coadsorbed state, the hydrogen bonding distance is 1.782 Å. In the transition state (TS6, O + H2O = 2OH), the forming O−H distance is 1.469 Å, and the critical parameters are rather close to those of the initial state, indicating an early transition state. The energy barrier is only 0.18 eV, close to the reported data (0.1532 and 0.18 eV39). The reaction is 26148



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Figure 8. Geometries and adsorption energy (eV) of 1MLH2O, 1/3MLOH, 1/2MLOH, 1MLOH, and 1MLO on Fe(100) surface (surface/blue; oxygen/red; hydrogen/yellow).

dissociation barriers are approximately constant at about 1 eV and the reaction is roughly thermo-neutral. For the third step, nO + H + OH → (n + 1)O + 2H, the barriers are close for n = 1−5 (0.70−1.00 eV), while increases to about 1.4 eV for n = 6, and to 1.68 eV for n = 7. For the last step, (n + 1)O + 2H → (n + 1)O + H2(g), the desorption energy varies in a range with the increased number of surface oxygen atoms for n = 1−6 and becomes close to zero for n = 7, indicating that the surface fully covered by oxygen atoms might not adsorb H2. It is also noted that as long as free adsorption sites are available for H2O adsorption, the H2O adsorption energies (about 0.45 eV) do not change too much with the increase of the adsorbed surface oxygen atoms. 3.6. Monolayer (ML) Coverage of H2O, OH, and O. Further we consider the ML coverage of H2O, OH, and O on the Fe(100) surface. The most stable adsorption configurations are shown in Figure 8. For (H2O)12, two structurally very different stable adsorption structures, on the basis of the fourmembered ring structures of (H2O)6 and the five-membered ring structure of (H2O)5, have very close adsorption energies (−6.43 vs −6.42 eV); in line with the results for n = 6−8. Five H2O molecules interact directly with the surface Fe atoms and have shorter Fe−O distances (2.212, 2.214, 2.243, 2.260, and 2.309 Å), while seven H2O molecules interacting with the first layer adsorbed water molecules have longer Fe−O distances (3.230, 3.251, 3.494, 3.599, 3.643, 3.951, and 3.989 Å). There are 12 shorter (1.670, 1.684, 1.694, 1.694, 1.733, 1.743, 1.757, 1.791, 1.794, 1.828, 1.866, and 1.879 Å) and four longer (2.006, 2.024, 2.142, and 2.477 Å) hydrogen bonding distances. For OH adsorption (Figure 8), we have computed different OH coverage. As shown in the Supporting Information, the average OH adsorption energy increases as the increase of the coverage (n = 1−6), indicating the intrinsic hydrogen bonding interaction between the adsorbed OH groups. For n = 2−4, the adsorbed OH groups form a well-ordered line structure at the bridge sites with tilted configuration; and each adsorbed OH group acts donor and acceptor and forms hydrogen bonding with the adjacent OH groups. At 0.5 ML coverage (n = 6), there are two parallel wellordered lines and the adsorbed OH groups do not share surface Fe atoms with the OH groups from the next lines; and this agrees with the observed surface structure of OH adsorption from LEED detection.33 Within one line, the distance of hydrogen bonding is 1.988 Å. Thermodynamically, the formation energy of 0.5 ML OH coverage is exothermic by 6.09 eV on the basis of six gaseous H2O molecules [6H2O(g) = 6OH(s) + 3H2(g)]. At 1 ML OH coverage (n = 12), all OH groups are at the bridge sites and form four parallel well-ordered lines and the adsorbed OH groups share surface Fe atoms with the adjacent

transfer from the bridge site to another bridge site, through a barrier of 0.44 eV (TS8), in order to dissociate easily. For the second OH dissociation (O + H + OH = 2O + 2H), the dissociation barrier is 0.98 eV. In the transition state (TS9), the O−H distance is 1.443 Å. In the final state, the O and H atoms are all at the hollow sites. The formation of gaseous H2 needs energy of 0.67 eV. As shown in Figure 6, the overall reaction for 2O + H2(g) is exothermic by 0.27 eV starting from the coadsorbed O + H2O or by −1.62 eV relative to gaseous H2O. The surface O assisted H2O dissociation has lower first H−O dissociation barrier (0.18 eV) than those of H2O dissociation (0.41 eV) and H2O mediated H2O dissociation (0.49 eV). Therefore, H2O can dissociate more easily on the O precovered Fe surface than on a clean or H2O-covered Fe surface. (b). H2O Dissociation on nO (n = 2−7) Precovered Fe(100) Surface. Starting from the 2O precovered surface with the formation of 2O + 2H2(g) (Figure 7), the first step of the third H2O dissociation is the coadsorption of two previously adsorbed O atoms and one H2O from gas phase (2O + H2O(g)), the computed H2O adsorption energy for 2O + H2O is −0.49 eV, which is slightly higher than that on the clean surface (−0.41 eV) and one O-precovered surface (−0.45 eV). For the kinetically preferred path (2O + H2O = O + 2OH), the computed barrier is 0.28 eV, and the forming O−H distance is 1.501 Å in the transition state (TS10). The dissociation is exothermic by 0.49 eV. For the subsequent dissociation steps (O + 2OH = 2O + H + OH; and 2O + H + OH = 3O + 2H); the first OH dissociation barrier is 0.88 eV, and the dissociation is exothermic by 0.35 eV. In the transition state (TS11), the O−H distance is 1.424 Å. Prior to the dissociation of the second OH, it is necessary to migrate to another adsorption site by rising 0.07 eV, and afterward the dissociation has a barrier of 0.70 eV, and the dissociation is exothermic by 0.49 eV. In the transition state (TS12), the breaking O−H distance is 1.400 Å. In the final state, the adsorbed O and H atoms are at the hollow sites. The formation of gaseous H2 needs energy of 0.75 eV (Figure 7), and the overall reaction for 3O + H2(g) is exothermic by 0.51 eV starting from the coadsorbed 2O + H2O or by −2.62 eV relative to gaseous H2O. As shown in Figure 7, further H2O dissociation on nO precovered surfaces (n = 3−7) has similar potential energy surfaces as found for that on 2O precovered surface, and we therefore do not wish to discuss their details. Instead we show their trend for general discussion and comparison. It is noted that the first step dissociation has always the lowest barrier nO + H2O → (n − 1)O + 2OH and is exothermic for n = 1−7, this step therefore does not control H2O dissociation. For the second step, (n − 1)O + 2OH → nO + H + OH, the 26149



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Figure 9. H2O and H2 desorption temperature under different conditions at 2.6 × 10−13 atmosphere on Fe(100) (s for surface species; and g for gaseous species).

eV on the basis of six gaseous H2O molecules [6H2O(g) = 6O(s) + 6H2(g)] or endothermic by 1.50 eV on the basis of 0.5 ML OH coverage [6OH(s) = 6O(s) + 3H2(g)]. In addition, the formation energy of 1 ML O coverage is exothermic by 5.22 eV on the basis of 12 gaseous H2O molecules [12H2O(g) = 12O(s) + 12H2(g)] or endothermic by 3.19 eV on the basis of 1 ML OH coverage [12OH(s) = 12O(s) + 3H2(g)]. Considering the formation of gaseous H2, it is thermodynamically possible to have 1 ML OH and O coverage on Fe(100). Kinetically, the formation 1 ML O coverage is strongly hindered, since the barrier of surface OH dissociation increases with the increase of surface O coverage. For example (Figure 7), the OH dissociation barrier on 7O precovered surface is 1.68 eV, which is much higher than those of 6O and 5O as well as 4O precovered surfaces (1.41, 1.32, and 0.85 eV, respectively). 3.7. Thermal Desorption of H2O and H2 on Fe(100) Surface. The equilibrium adsorption configurations depend on coverage and mutual interaction of the adsorbed surface species as well as temperature. In this part we will discuss desorption of H2O and H2 at different temperatures and a certain pressure

lines; and the hydrogen bonding distance is 1.975 Å. Despite the hydrogen bonding interaction, the average adsorption energy (−4.01 eV) for n = 12 is lower than that of one OH (−4.14 eV), in opposite with those for n = 2−6 (−4.27−4.34 eV). This difference can be attributed to the repulsive interaction of the OH groups sharing surface iron atoms. Thermodynamically, the formation energy of 1 ML OH coverage is exothermic by 8.41 eV on the basis of 12 gaseous H2O molecules [12H2O(g) = 12OH(s) + 6H2(g)]. Kinetically, the formation of 1 ML OH coverage is also possible since OH can be formed from H2O direct dissociation or much easily from O-assisted H2O dissociation [nO + H2O = (n − 1)O + 2OH] as shown in Figure 7. For the ML coverage of atomic oxygen, we used the (3 × 4) model, which is different from the previous study80 using a (1 × 1) surface cell for 1 ML coverage. As shown in Figure 8, all O atoms are at the hollow sites; and the Fe−O distances are 2.053 Å. On the basis of gaseous O2, the calculated adsorption energy per O atom is −2.97 eV, which is slightly lower than the previously reported result (−3.09 eV). Thermodynamically, the formation energy of 0.5 ML O coverage is exothermic by 4.59 26150



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the O [O(s)+2H(s) → O(s) + H 2 (g)] and OH [2OH(s)+2H(s) → 2OH(s) + H2(g)] precovered surfaces (Figure 9b), the computed desorption temperature is 210 and 230 K, respectively, and they are close to the reported 200 K. We further increased the surface oxygen coverage [nO(s)] + 2H(s) → nO(s) + H2 (g), n = 2−8], and the computed H2 desorption temperatures (Figure 9d) are in line with the desorption energies of H2, which depends on surface structure and the adsorption sites of 2H, and lower than the experimentally observed 300−312 K. Since the desorption temperatures of H2O from OH disproportionation (310 K) and the distinct desorption peak of H2 (300−312 K) coincide and they are much less coverage dependent, in sharp contrast to those of H2 desorption on clean iron surfaces,81 the detected H2 desorption peak comes from OH disproportionation [2OH(s) → 2O(s)+ H2(g)]. Indeed, the computed barriers for OH dissociation and disproportionation are very close (0.85 and 0.74 eV, respectively). Therefore, the kinetics of these two reactions are decisive in determining the desorption temperature of H2O and H2. Despite of the excellent agreement between theory and experiment in H2O and H2 adsorption and desorption kinetics and thermodynamics, we are interested in the long-range dispersion corrections for van der Waals interaction of adsorbed H2O and H2 on the Fe(100) surface. In our calculations we have used the Grimme semiempirical GGAtype functional (PBE-D2). As listed in Table 6, the desorption

(Figure 9). On the basis of Gibbs free energy changes of the related reaction at different temperature, one can estimate the reaction temperature at which the reaction occurs. Since the experiment was performed under ultrahigh vacuum condition for water adsorption on the Fe(100) surface, so we used the data at 2.6 × 10−13 atm (2 × 10−10 Torr33) for discussion and comparison. The adsorption of H2O on Fe(100) surface has been investigated by Hung et al.33 The TPD spectra show that water desorbs from three states at 165, 220, and 310 K. The peak at 165 K is assigned to desorption of water molecule having only hydrogen bonding with other water molecules. The peak at 220 K is desorption of molecular water chemisorbed on the Fe(100) surface. When the temperature is heated near 310 K, the disproportionation of OH species occurs and forms molecular H2O and surface O atoms. In addition, H2 desorption at 300−312 K along with a shoulder at about 380 K was observed. Two small H2 peaks at 200 and 160 K were also observed. On the clean Fe(100) surface, Bozso et al.81 found two discernible states (β1 and β2). The β1 state has temperature maximum at about 300 K, whereas the β2 state shifts toward higher temperature (430− 400 K) with increasing coverage. In our previous study,59 we found one broad desorption state with temperature maximum at about 310 K, which is very close to the experimentally detected β1 state. However, we could not find the temperature maximum of the β2 state. The computed desorption temperature of one water molecule on the clean surface [H2O(s) → H2O(g)] is 240 K (Figure 9a), which is close to the experimentally detected 220 K. The computed desorption temperature of one water molecule formed from OH disproportionation [2OH(s) → O(s)+ H2O(g)] is 315 K (Figure 9b), in excellent agreement with the observed 310 K. However, the desorption temperature of one water molecule on one oxygen precovered surface [O(s)] + H2O(s) → O(s) + H2O(g)] is 255 K (Figure 9b), which is more close to that on the clean surface than to that from OH disproportionation. This clearly indicates that the determining factor for desorption peak is the kinetic effect of OH disproportionation at 310 K rather only H2O desorption. To verify this proposal, we further increased surface oxygen coverage [nO(s)] + H2O(s) → nO(s) + H2O(g), n = 2−7. Depending on the hydrogen bonding interaction between the adsorbed O atoms and H2O molecule (Figure 9c), the computed desorption temperature is in the range of 220−260 K, lower than the observed 310 K from OH disproportionation. In addition, we computed the stepwise desorption of the adsorbed (H2O)4 cluster (Figure 9a), which has three water molecules interacting mainly with the surface iron atoms, and one water molecule dominantly interacting via hydrogenbonding with other water molecules. As shown in Table 4, the stepwise adsorption energy increases from −0.41 eV for the first H2O molecule to −0.52 and −0.54 eV for the second and third H2O molecules, respectively, as well as to −0.63 eV for the fourth H2O molecule. In turn, the temperature of stepwise desorption decreases from 290 K for the fourth H2O molecule to 270 and 265 K for the third and second H2O molecules, respectively, and to the first H2O molecule (240 K). All these indicate the kinetically controlled H2O desorption at 310 K from OH disproportionation. For H2 desorption on the clean surface [2H(s) → H2(g)], the computed desorption temperature is 315 K, in agreement with the experimental data for the β1 state at about 300 K.81 On

Table 6. Desorption Temperature (K) and Desorption Energies (eV, Negative of Adsorption Energies) of H2O and H2 under Different Conditions at 2.6 × 10−13 Atmosphere on the Fe(100) Surface from PBE, PBE-D2 Theory, and Experiment Available Data H2O(s) → H2O(g) 2OH(s) → O(s) + H2O(g) 2H(s) → H2(g)

PBE

PBE-D2

ref 33

240 K [0.41 eV] 315 K [0.72 eV] 315 K [0.89 eV]

280 K [0.58 eV] 375 K [0.98 eV] 385 K [1.11 eV]

220 K 310 K 300 K

temperature of one adsorbed H2O molecule on the clean surface from PBE-D2 [H2O(s) → H2O(g)] is 280 K, which is higher than that of PBE (240 K) and the experimental value (220 K); and the corresponding desorption energy from PBED2 is also higher than that of PBE (0.58 vs 0.41 eV). For the disproportionation desorption [2OH(s) → O(s) + H2O(g)], the temperature after PBE-D2 (375 K) is higher than that of PBE (315 K) and the experimental value (310 K); and the corresponding desorption energy is also higher (0.98 vs 0.72 eV). For H2 on the clean surface [2H(s) → H2(g)], the desorption temperature and energy from PBE-D2 (1.11 eV and 385 K) are higher than that from PBE (0.89 eV and 315 K); and the desorption temperature from PBE is very close to the experimental value (300 K). All these show clearly that dispersion corrections overestimate the dispersion interactions of not only the weakly adsorbed H2O molecule but also the strongly adsorbed H2 molecule on the Fe(100) surface. Actually these effects have also been found for the adsorption of azobenzene at the coinage M(111) (M = Cu, Ag, Au) surfaces,82 Ar, Kr and Xe on the Pt(111), Pd(111), Cu(111), and Cu(110) surfaces.83 In addition, it is reported that the 26151



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interaction in the layered graphite, hexagonal-born nitride structures is also overestimated.84 The calculated thermal desorption temperatures of H2O and H2 on the clean surface agree with the available experimental data, and precovered O species affect only slightly H2O desorption temperatures. The characteristic desorption temperatures of H2O and H2 coincided at near 310 K is the kinetic effect of OH disproportionation rather only. The interplay of the thermodynamics between experiment and theory provides a novel characterization tool for investigating surface properties and active facets of catalyst systems.

atures of H2O and H2 coincided at near 310 K come from OH disproportionation (2OH → O + H2O) and dissociation (2OH → 2O + H2) and therefore represent a kinetic controlled phenomena. These results provide insights into the waterinvolved catalytic reactions catalyzed by iron particularly and broaden our fundamental understanding of water interaction with metal surfaces generally. It is finally noted that dispersion corrections (PBE-D2) overestimate the adsorption energy of H2O and H2 on the clean Fe(100) surface to a large extent.

4. CONCLUSIONS In this work, we have investigated the H, O, OH, and H2O adsorption, as well as H2O aggregation and dissociation on the Fe(100) surface on the basis of density functional theory calculations and ab initio atomistic thermodynamics. On the clean surface at the lowest coverage, the adsorption of atomic hydrogen has very close adsorption energy at the bridge and hollow sites, and atomic oxygen prefers the hollow site strongly. Hydroxyl prefers the bridge, and H2O prefers the top site. For the adsorption of (H2O)n clusters (n = 2−8), the adsorption energy is determined by the direct H2O to surface Fe interaction and hydrogen bonding. For n = 1−3, direct H2O to surface Fe interaction is dominant, and hydrogen bonding becomes more important for n ≥ 4. For n = 6−8, 12, structurally very different adsorption configurations have very close adsorption energies. Monomeric H2O dissociation needs an energy barrier of 0.41 and 0.85 eV into OH + H and O + 2H, respectively; however, H2O dissociation barrier on H2O (0.49 eV) and OH (0.93 eV) precovered surface is higher than that on the clean surface. Oassisted H2O dissociation is favorable kinetically (O + H2O = 2OH), and further OH dissociation is roughly thermo-neutral. With surface O coverage increase, further H2O dissociation on nO precovered surfaces (n = 2−7) has similar potential energy surfaces; and H2 desorption energy varies in a range with the increased number of surface oxygen atoms for n = 1−6 and becomes close to zero for n = 7, indicating that the surface fully covered by oxygen atoms might not adsorb H2. As long as free adsorption sites are available, H2O adsorption does not change too much with the increase of the adsorbed surface oxygen atoms. For OH adsorption at different coverage, the adsorbed OH groups at the bridge sites form very ordered line structures and each adsorbed OH group as donor and acceptor forms hydrogen bonding with the adjacent OH groups within each line. At 0.5 ML coverage (n = 6), the adsorbed OH groups at the bridge sites do not share surface iron atoms and form two well-ordered parallel lines; this agrees with the experimentally observed surface structures from low-energy electron diffraction. At 1 ML coverage (n = 12) the adsorbed OH groups at the bridge sites share surface iron atoms and form four parallel lines. For 1 ML adsorbed O atoms, all O atoms are located at the most stable hollow sites. Energetic analysis reveals that the formation of 1 ML OH coverage is accessible both kinetically and thermodynamically, while that of 1 ML O coverage is hindered kinetically since the OH dissociation barrier increases strongly with the increase of O pro-covered coverage. The calculated thermal desorption temperatures of H2O and H2 on the clean surface agree with the available experimental data, and precovered O species affect desorption temperatures of H2O only slightly. The characteristic desorption temper-

Model test for Fe(100) on H2O adsorption (Table S1); adsorption energies and bond distances of IS, TS, and FS for the dissociations on Fe(100) (Tables S2−4); adsorption energies and desorption temperature of H2O and H2 under some different conditions on Fe(100) from theory and experiment (Table S5); optimized geometries for the stationary points of the dissociations on Fe(100) (Figures S1−S9); the stable geometries of nOH at different coverage on Fe(100) (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.

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ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2011CB201406), the National Natural Science Foundation of China (Nos. 21273262 and 21273266), and the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge general financial support from the BMBF and the state of Mecklenburg-Vorpommern.

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REFERENCES

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High Coverage Water Aggregation and Dissociation on Fe(100): A

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