Coverage Dependent Water Dissociative Adsorption on the Clean and

Article pubs.acs.org/JPCC

Coverage Dependent Water Dissociative Adsorption on the Clean and O‑Precovered Fe(111) Surfaces 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, China ∥ Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany ‡

S Supporting Information *

ABSTRACT: Water dissociative adsorption on the clean and Oprecovered Fe(111) surfaces at different coverage have been studied using the density functional theory method (GGA-PBE) and ab initio atomistic thermodynamics. On the clean p(3 × 3) Fe(111) surface, surface H, O, OH, and H2O species can migrate easily. Considering adsorption and H-bonding, the adsorbed H2O molecules can be dispersed or aggregated in close energies at low coverage, while in different aggregations at high coverage, indicating that the adsorbed H2O molecules might not have defined structures, as observed experimentally. On the O-precovered surface (nO = 1−8), the first dissociation step, nO + H2O = (n − 1)O + 2OH, has a very low barrier and is reversible; and the barriers of the sequential OH dissociation steps, (n − 1)O + 2OH = nO + H + OH and nO + H + OH = (n + 1)O + 2H, are close (0.9−1.2 eV). All of these barriers are coverage independent. For OH and H adsorption at 1/3 ML coverage, surface OH forms a trimer (OH)3 unit, and surface O forms a regular linear pattern. At one ML coverage, there are three dispersed (OH)3 units for OH adsorption and three well-ordered parallel lines for O adsorption. The average adsorption energies for OH and O adsorption are coverage independent. Desorption temperatures of H2O and H2 under ultrahigh vacuum conditions are computed. Systematic comparison among the Fe(110), Fe(100), and Fe(111) surfaces reveal their intrinsic differences in water dissociative adsorption and provide a basic understanding of water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces. study, Kalz et al.17 investigated the influence of an electron beam on H2O adsorption on the Fe(111) surface under ultrahigh vacuum (UHV) conditions. They found that the coverage limit is in the range of 0.5−0.9 ML at 300 K as well as 0.2−0.3 ML at 620 K without electron irradiation. At 300 K, an additional adsorption peak of surface OH groups is observed under electron irradiation, while no marked influence of an electron beam on water adsorption was found at 620 K under similar conditions. Müssig and Arabczyk18 studied water adsorption on the Fe(111) surface using low energy electron diffraction and work function changes under UHV conditions. They found a random adsorption layer, which was transferred into a p(1 × 1) structure when heated between 473 and 673 K. Jiang et al.19 studied water adsorption and dissociation on clean and gallium precovered Fe(111) surfaces by using Auger electron spectroscopy and thermal energy atom scattering. They found that water adsorption on the surface occurs via a molecular precursor state and that a passivated overlayer is

1. INTRODUCTION The rich molecular properties of water and its appearance in many interfacial systems make it one of the most interesting and important adsorbates.1 The interaction of water with surfaces is a focus of research in a surprisingly wide variety of scientific disciplines.2 The interaction of water and iron, which forms iron oxides, is one of the important processes in nature. Catalytic water conversion plays an important role in many industrial processes, such as the water−gas shift reaction generating hydrogen for a wide range of applications and Fischer−Tropsch synthesis converting gas into hydrocarbons as chemicals and fuels.2 In these reactions, iron-based catalysts have been widely used. Understanding the interaction of water and iron as well as the corrosion of iron-based materials is of technological importance. The fundamental aspect is the elementary steps at the atomic or molecular level, and the initial stages involve water adsorption on surfaces and water dissociation into H + OH as well as O + 2H. In contrast to the plentiful experimental and theoretical studies about water adsorption and dissociation on the Fe(110) and Fe(100) surfaces,3−16 only a few reports about these reactions on the Fe(111) surface are known.13,17−19 In an early © 2015 American Chemical Society

Received: March 9, 2015 Revised: April 30, 2015 Published: May 4, 2015 11714

DOI: 10.1021/acs.jpcc.5b02297 J. Phys. Chem. C 2015, 119, 11714−11724

Article

The Journal of Physical Chemistry C

Figure 1. Surface structures of Fe(111) in (a) top and (b) side views, and possible adsorption sites: on-top (t), shallow-hollow (sh), deep-hollow (dh), and the quasi-4-fold hollow (qff) sites; and the most stable adsorption configuration and energy of (c) H, (d) O, (e) OH, and (f) H2O.

μB.28 To enable direct and systematic comparisons, we used the same convergence parameters as those used on the Fe(100) and Fe(110) surfaces in our previous studies.15,16 Our previous studies showed that PBE is very well applicable in studying the adsorption and dissociation as well as desorption of H2O,15,16 H2,29 and CO30,31 on different Fe surfaces. Particularly as shown in Table 1 in section 4 below, the PBE computed energetic data of adsorption and dissociation of H2O on the Fe(100) surface agree very well with the results of (RPA+EX) @PBE.14 2.2. Model. For the Fe(111) surface, a periodic slab with a vacuum region with 15.0 Å width was used to separate the repeating slabs. In order to choose a reasonable slab model for H2O adsorption, we tested the effects of the number of relaxation layers, k-points, and surface size on H2O adsorption energy, and all of these testing results are listed in the Supporting Information (Table S1). On the basis of these tests, the surface structural relaxation and the total energy calculation were performed with 3 × 3 × 1 Monkhorst−Pack k-point sampling. The p(3 × 3) surface size was used. A six-layer model was used, where the first three layers including adsorbates were relaxed, and the bottom three layers were fixed. The structure includes 54 Fe atoms. The top and side views and possible adsorption sites of the Fe(111) surface are shown in Figure 1. The adsorption energy was calculated by using the expression defined as Eads = EX/slab − Eslab − EX, 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 adsorbates in gas phase. Therefore, the more negative the Eads, the stronger the adsorption. The barrier (Ea) and the reaction energy (ΔER) are calculated according to Ea = ETS − EIS and ΔER = EFS − EIS, where EIS, ETS, and EFS are the energies of the corresponding initial state (IS), transition state (TS), and final state (FS), respectively. It is noted that the reported 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.29,31,32

formed on the clean surface exposed to water at 423 K, preventing the further oxidation of the sample. At 423 K, there are only O atoms on the surface, although there still exists equilibrium between gas phase water and the molecular precursor water, and the residence time of water is so short that the steady state of the molecular precursor water is low. When the temperature is up to 700 K, surface oxygen can dissolve into the bulk. The only theoretical study about the adsorption and dissociation of a single water molecule on the Fe(111) surface is reported by Lazar et al.,13 on the basis of a generalized gradient scheme (PW91) and range-separated hybrid functional (HSE06) methods of the density functional theory. They found that the surface coadsorbed H and OH represent a deep minimum on the potential energy surface by using HSE06. They also proposed the potential role of surface OH in the reactivity of iron nanoparticles. As our continuing interests in studying the interaction mechanisms of H2O with iron surfaces, we computed systematically H2O adsorption and dissociation on the clean as well as partially oxidized Fe(111) surfaces. Our main goals are the preferred adsorption sites; coverage dependent adsorption structures on the clean surface; surface oxygen mediated H2O dissociation; and desorption of H2O and H2 molecules. This enables a direct comparison of H2O adsorption between the Fe(111) as well as the Fe(100) and (110) surfaces from our previous studies.15,16

2. METHODS AND MODELS 2.1. Method. All calculations were performed with the plane wave pseudopotential code in the Vienna ab initio simulation package (VASP).20,21 The electron−ion interaction is described with the projector augmented wave (PAW)22,23 method. Exchange and correlation energies were described using the spin-polarized generalized gradient approximation and Perdew−Burke−Ernzerhof functional (GGA-PBE).24 Spinpolarized calculations were performed to account for the magnetic properties of iron. Transition state structures were estimated using the climbing image nudged elastic band method (CI-NEB).25 For each optimized stationary point, vibrational analysis was performed at the same level of theory to determine its character (minimum or saddle point). The optimized lattice parameter was calculated using the bodycentered cubic (bcc) unit cell, and its reciprocal space is sampled with 15 × 15 × 15 k-point grid generated automatically using the Monkhorst−Pack method.26 The optimized lattice constant of 2.835 Å is close to the experimental value of 2.866 Å.27 The calculated magnetic moment of 2.226 μB is close to the experimental value of 2.22

3. RESULTS AND DISCUSSIONS 3.1. Adsorption of H, O, OH, and H2O. Figure 1 shows the possible adsorption sites, i.e., the top (t) site of one first layer iron atom (Fe1), the shallow-hollow (sh) site from one second layer and three first layer iron atoms (Fe2 + 3Fe1), the deep-hollow (dh) site from three first layer, three second layer, and one third layer iron atoms (3Fe1 + 3Fe2 + 1Fe3), and quasi-4-fold (qff) hollow site from two first layer, one second layer, and one third layer iron atoms (2Fe1 + 1Fe2 + 1Fe3). For the adsorption of H, O, OH, and H2O, we considered 11715


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Figure 2. Most stable water monomer and small clusters adsorbed on the Fe(111) surface (Fe/blue; O/red; and H/yellow).

urations by considering H2O adsorption and H-bonding for two water molecules. In the first one, (H2O)2-t1, both H2O molecules are adsorbed on the top sites of two neighboring first layer iron atoms, and the Fe−O distances (2.134 and 2.150 Å, respectively) are close to that of one H2O adsorption (2.135 Å). The adsorption energy of (H2O)2-t1 is −1.17 eV, which is equal to twice that of one H2O adsorption. Since the distance of two neighboring first layer iron atoms is very long (4.010 Å), it is not possible to form effective H-bonding between two water molecules (2.966 Å) at top sites, and therefore, no significant H-bonding stabilization can be expected. In the second one, (H2O)2-t2, one H2O molecule is adsorbed on the top site with the Fe−O distance of 2.063 Å, shorter than that of one H2O adsorption (2.135 Å). The second H2O molecule forms H-bonding with the first adsorbed H2O molecule with the H−O distance of 1.652 Å. It is noted that the second H2O molecule is located over the deep-hollow site and has one H atom pointing to the third layer iron atom with the Fe−H distance of 3.204 Å, and the shortest distance between the O atom and first layer as well as the second layer iron atom is 3.350 and 3.842 Å, respectively. This reveals that it is not possible to form effective adsorption interaction of the second H2O molecule and the surface. The computed adsorption energy is −1.15 eV, which is only 0.02 eV lower than that of the first one, and therefore, both dispersed and aggregated adsorptions are possible. On the basis of (H2O)2, we computed two adsorption configurations for (H2O)3. In the first one, (H2O)3-t1, all H2O molecules are adsorbed at the t sites, and the Fe−O distance is 2.152, 2.154, and 2.165 Å, respectively. In the second one, (H2O)3-t2, two H2O molecules are located at the top sites with

different initial adsorption configurations, and the obtained most stable adsorption configurations are given in Figure 1. The adsorption energy and structure parameters are listed in the Supporting Information (Table S2). As shown in Figure 1, the most stable adsorption sites for H, O, and OH are the quasi-4-fold hollow sites, while that for H2O is the top site. For H, O, and OH, the computed adsorption energy is −0.46, −2.92, and −3.93 eV, respectively. The adsorbed H and O atoms as well as the OH group on the surface can move easily from one quasi-4-fold hollow site to another one via the shallow-hollow (sh) site with barrier of 0.03, 0.27, and 0.45 eV, respectively. For the diffusion of H atom, the result is in agreement with that found by Huo et al.33 that H atom is mobile on the Fe(111) surface. The computed H2O migration barrier from one top site to another top site is 0.48 eV. Therefore, all four surface species can migrate easily on the Fe(111) surface; and this will affect the high coverage adsorption configurations. 3.2. Adsorption of (H2O)n on Fe(111). Because of the interaction of H2O with the iron surface and H-bonding among adsorbed H2O molecules, we used sequential adsorption to find the stable adsorbed H2O clusters on the iron surface. The most stable adsorption configurations of H2O clusters (n = 1−9) are shown in Figure 2. The computed adsorption energies and selected bond parameters are listed in the Supporting Information (Table S3). The computed structure parameters of (H2O)n adsorbed over the deep hollow sites with one H atom pointing to the surface are shown in the Supporting Information (Table S4). Because of the very open surface structure and the three very close surface layers, we computed two adsorption config11716


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Figure 3. Optimized geometries for the stationary points in the reaction of H2O direct dissociation (Fe/blue; O/red; and H/yellow).

hollow sites; and each H2O molecule works as a H donor and acceptor for H-bonding. The most stable adsorption configuration is (H2O)6-t2, and the adsorption energy is −3.86 eV, higher than those (−3.55 and −3.76 eV, respectively) of (H2O)6-t1 and (H2O)6-t3 as well as 6-fold that of one H2O adsorption (−3.48 eV). This indicates that the aggregated forms are more stable than the dispersed one and that the structure with a five-membered ring is more stable than that with the six-membered ring. As shown in (H2O)4−6, aggregated adsorption configurations having H-bonding are more stable than dispersed ones at the top sites without H-bonding, we showed only the results of (H2O)7−9 having H-bonding. On the basis of (H2O)6-t2 and (H2O)6-t3, we computed two adsorption configurations for (H2O)7. The first one, (H2O)7-t1, deduced from (H2O)6-t2, has two exocyclic H2O molecules at the top sites of two neighboring iron atoms. The second one, (H2O)7-t2, deduced from (H2O)6-t3, has one exocyclic H2O molecule at the top site. The first one has higher adsorption energy than the second one (−4.56 vs −4.42 eV), indicating that the structure with a five-membered ring is more stable than that with the sixmembered ring. Furthermore, we computed two adsorption configurations for (H2O)8 on the basis of (H2O)7-t1 and (H2O)7-t2. Both adsorbed configurations, (H2O)8-t1 and (H2O)8-t2, have very close adsorption energies (−5.20 vs −5.17 eV), indicating that both aggregated forms are possible. On the basis of (H2O)8, we computed two adsorption configurations for (H2O)9, (H2O)9t1 with fused five-membered and six-membered rings and (H2O)9-t2 with three exocyclic H2O molecules perfectly remotely located around the six-membered ring without Hbonding. The first one has lower adsorption energy than the second one (−5.67 vs −6.00 eV), indicating that the structure with only a six-membered ring is more stable than that with fused five- and six-membered rings. On the basis of these results, it is clear that the adsorbed H2O molecules can be dispersed or aggregated at low coverage in close adsorption energies while being aggregated at high coverage. For the aggregated adsorption configurations at high coverage, the structures with five-membered and six-membered rings have close adsorption energies. This shows that the adsorbed H2O molecules on the Fe(111) surface might not have defined structures, as observed experimentally. 3.3. H2O Dissociation on the Clean Fe(111) Surface. On the basis of the most stable adsorbed H2O structure at the top site, H2O dissociation into surface H + OH as well as O + 2H was calculated. The optimized structures of the IS, TS, and FS are shown in Figure 3, and the structural parameters are listed in the Supporting Information (Table S5). The reaction

the Fe−O distances of 2.049 and 2.229 Å, while the middle H2O molecule is located over the deep-hollow site and has one H atom pointing to the third layer iron atom with the Fe−H distance of 3.372 Å. In addition, the middle H2O molecule forms H-bonds with one top site H2O molecule by accepting the H atom with a H-bonding distance of 1.606 Å and with another top site H2O molecule by providing a H atom with a H-bonding distance of 2.014 Å. Despite their very different adsorption configurations, they have the same adsorption energy (−1.79 eV), which is slightly higher than 3-fold of one H2O adsorption (−1.74 eV). This indicates the possibility of both dispersed and aggregated adsorptions. On the basis of (H2O)3, we computed two adsorption configurations for (H2O)4. In the first one, (H2O)4-t1, all H2O molecules are adsorbed at the top sites, and the Fe−O distance is 2.146, 2.157, 2.166, and 2.175 Å, respectively. On the basis of (H2O)3-t2, the fourth H2O molecule is located at the top site and provides one H atom to the middle H2O molecule for Hbonding. In the star-like (H2O)4-t2, three H2O molecules are adsorbed at the top sites with the Fe−O distances of 2.088, 2.112, and 2.259 Å, respectively, and the center H2O molecule has one H atom pointing to the surface. The three H-bonding distances are 1.727, 1.815, and 1.972 Å, respectively. The adsorption energy of (H2O)4-t2 is −2.52 eV, higher than that of (H2O)4-t1 (−2.38 eV) as well as that of 4-fold of one H2O adsorption (−2.32 eV). This indicates that the aggregated form is more preferred over the dispersed one. We computed two adsorption configurations for (H2O)5. In the first one, (H2O)5-t1, all H2O molecules are adsorbed at the top sites, and the Fe−O distance is 2.160, 2.166, 2.170, 2.170, and 2.180 Å, respectively. In the second one, (H2O)5-t2, each H2O molecule works as a H donor and acceptor for H-bonding, and three H2O molecule are adsorbed on the top sites with the Fe−O distances of 2.122, 2.124, and 2.210 Å, while each of the other two H2O molecules has one H atom pointing to the deep-hollow site. The adsorption energy of (H2O)5-t2 is −3.21 eV, which is higher than that of (H2O)5-t1 (−2.99 eV) as well as 5-fold that of one H2O adsorption (−2.90 eV). This also indicates that the aggregated form is more preferred over the dispersed one. For the adsorption of (H2O) 6, we computed three adsorption configurations. In the first one, (H2O)6-t1, which is deduced from (H2O)5-t1, all six H2O molecules are adsorbed on the top sites. In the second one, (H2O)6-t2, which is deduced from (H2O)5-t2, has a five-membered ring with an exocyclic H2O molecule adsorbed at the top site. The third adsorption configuration, (H2O)5-t3, has cyclic hexamer structure with alternating H2O molecules at the top sites and also H2O molecules having an H atom pointing to the deep11717


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Figure 4. Total potential energy surfaces for the reaction of Fe + 9H2O(g) = 9O/Fe + 9H2(g). The red data in parentheses are the relevant reaction barriers (s for surface species; and g for gaseous species). 11718


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molecule after the formation of H2(g), i.e., nO + H2O(g) = (n + 1)O + H2(g) (n = 1−8). The optimized structures for the stationary points of IS, TS, and FS are given in the Supporting Information (Figures S2−S9). The bond parameters of IS and FS are given in the Supporting Information (Table S7). The dissociation barriers, dissociation energies, and critical bond distances of TS are given in the Supporting Information (Table S8). The potential energy surface is shown in Figure 4. The model begins with the coadsorption of one surface O (θO = 0.11 ML, per Fe atom in the first layer) and one gaseous H2O(g) by considering H-bonding. The adsorption energy of H2O is 0.91 eV, which is higher than that (−0.58 eV) on the clean surface, and this is due to the H-bonding (1.535 Å) in the coadsorbed initial state O + H2O(s). The following step is the O assisted H2O dissociation into the coadsorbed OH + OH, where one OH is located at the quasi-4-fold hollow site, and another one is located tilted at the top site; the H-bonding distance is 1.717 A. In the transition state TS4, the breaking O−H distance is 1.310 Å. The dissociation barrier is only 0.02 eV, and the reaction energy is slightly exothermic by 0.10 eV. Note that this reaction is the reverse disproportionation of the coadsorbed OH groups, 2OH = O + H2O(s), which has reaction barrier of 0.12 eV and is endothermic by 0.10 eV. These energetic data reveal the fast equilibrium of this reversible reaction. For OH + OH following dissociation into O + H + OH, there are two possible routes, i.e., the direct one and the stepwise one. In the direct route, the barrier is as high as 1.72 eV. In the stepwise route, the first step is the migration of the OH group tilted at the top site moving to the quasi-4-fold hollow site by breaking the H-bond. The barrier via the transition state TS5 is 0.25 eV, and this step is endothermic by 0.11 eV. After the migration, both OH groups are located at the quasi-4-fold hollow site. The second step is the dissociation of one of OH group. In the transition state TS6, the breaking O− H distance is 1.399 Å. The barrier is 0.95 eV, and this step is exothermic by 0.01 eV. Direct comparison shows that the stepwise route is more preferred kinetically by 0.66 eV. For the dissociation of the second OH group, the breaking O−H distance in the transition state TS7 is 1.408 Å. The barrier is 1.04 eV, and the reaction is exothermic by 0.33 eV. The total reaction energy is exothermic by 0.33 eV. As shown in Figure 4, the formation of gaseous H2 is endothermic by 0.81 or 0.48 eV on the basis of the coadsorbed O + 2H or coadsorbed O + H2O, respectively while being exothermic by 0.43 eV on the basis of gaseous H2O. This shows the very high similarity between the clean surface and one Oprecovered surface. Since the potential energy surfaces for further H2O dissociations on the nO-precovered surface Fe(111) (n = 2− 8, θO = 0.22−0.89 ML) are similar to that on one O-precovered surface (Figure 4), we show their trends for general comparison (Figure 5). It shows that the H2O adsorption energies, Eads [nO + H2O(g) = nO + H2O(s)], are in the range of 0.86−0.99 eV, very close to that on one O-precovered surface (−0.91 eV). In the coadsorbed state [nO + H2O(s)], the adsorbed H2O molecule forms H-bonding with the surface O atom, and the Hbonding distances are in the range of 1.5−1.7 Å, close to that on one O-precovered surface (1.535 Å). Apart from the adsorption energies and H-bonding distances, very high similarity is also found for the dissociation barriers and reaction energies between the nO-precovered surfaces and one O-precovered surface. For example, the barriers of the first

barriers, the reaction energies, and the structural parameters of the TS are shown in the Supporting Information (Table S6). The full reaction potential energy surface is shown in Figure 4. In the transition state (TS1) of the first O−H bond dissociation, the O atom is at the top site, and the H atom is heading to one second layer Fe atom. The breaking O−H distance is 1.404 Å, and the forming Fe−H distance is 1.844 Å. In the coadsorbed state (OH + H), the OH group is located at the quasi-4-fold hollow site with the Fe−O distances of 2.096 and 2.271 Å, and the H atom is located at the deep-hollow site with the Fe−H distance of 1.628 Å to one second-layer iron atom. The activation energy is 0.54 eV, and the reaction is exothermic by 0.51 eV. Starting from the coadsorbed OH and H, we computed OH dissociation into O and H atoms. In the transition state (TS2), the O atom is located at the quasi-4-fold hollow site with the Fe−O distance of 1.947 Å to the second layer iron atom; and the H atom is toward one second layer Fe atom with the Fe−H distance of 1.681 Å; and the breaking O−H distance is 1.384 Å. In the coadsorbed state (O + 2H), the O and H atoms are located at their most stable quasi-4-fold hollow sites. The activation energy is 0.93 eV, and the dissociation reaction is slightly endothermic by 0.10 eV. As shown in Figure 4, the formation of gaseous H2 is endothermic by 0.60 or 0.19 eV on the basis of coadsorbed O + 2H or adsorbed H2O, respectively, while being exothermic by 0.39 eV with respect to gaseous H2O. Apart from the dissociative adsorption of the monomeric H2O molecule, we also computed H2O mediated H2O dissociation of the (H2O)2 cluster. There are three steps for H2O mediated H2O dissociation, i.e., (i) (H2O)2 = OH + H + H2O and (ii) OH + H + H2O = O + 2H + H2O or (iii) OH + H + H2O = 2OH + 2H. The bond parameters of the stationary points of IS and FS are listed in the Supporting Information (Table S5). The optimized geometries of these points are shown in the Supporting Information (Figure S1). The dissociation barriers, dissociation energies, and critical bond distances of TS are given in the Supporting Information (Table S6). The total potential energy surface is shown in Figure 4. Since there is no H-bonding stabilization in the dispersed (H2O)2-t1, no significant effect from the coadsorbed H2O can be expected. Indeed, the computed dissociation barriers and energies are nearly the same as those found for monomeric H2O dissociation. The detailed information on the (H2O)2-t1 dissociation is listed in the Supporting Information (Tables S5 and S6, and Figure S1). For the aggregated dimeric (H2O)2-t2 with H-bonding stabilization, we computed the dissociation of the H2O molecule adsorbed at the top site. In the first step, (H2O)2 = OH + H + H2O, the dissociation barrier via the transition state TS3 is 1.00 eV, which is 0.40 eV higher than that of the dissociation of (H2O)2-t1, and the reaction is exothermic by 0.57 eV. This process is kinetically less favorable and was not further considered. Although both dispersed and aggregated states of (H2O)2 are very close in energy, the dissociation of the dispersed adsorbed H2O molecules is more favored kinetically. Therefore, we paid attention only to the stepwise H2O dissociative adsorption on the O-precovered surface after the formation of gaseous H2 as discussed below. 3.4. H2O Dissociation on the O-Precovered Fe(111) Surface. On the O-precovered surface, H2O dissociative adsorption is modeled with sequential increase of H2O 11719


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Figure 5. H2O adsorption energy Eads (eV), the barrier energies Ea1 (eV) for nO + H2O = (n − 1)O + 2OH, and Ea2, Ea3 (eV) for the OH dissociation in the reactions of (n − 1)O + 2OH = nO + H + OH and nO + H + OH = (n + 1)O + 2H and H2 desorption energies (Edes, eV) in the reaction of (n + 1)O + 2H = (n + 1)O + H2(g).

Figure 6. Adsorption structures and energies of OH and O at different coverages on the Fe(111) surface (Fe/blue; O/red; and H/yellow).

step dissociation, nO + H2O = (n − 1)O + 2OH, are also very low (0.01−0.06 eV) for n = 2−8 (θO = 0.22−0.89 ML), and the reactions are exothermic by up to 0.12 eV. These energetic data reveal fast equilibrium of this reversible reaction, independent of the number of surface oxygen atoms. Since the barrier of one H2O dissociation on the Fe(111) surface is very low, it can be deduced that on O-precovered surfaces with available sites for further H2O adsorption, H2O can easily dissociate into OH + H, independent on the number of precovering O atoms. Indeed, such a dissociation process has been identified experimentally by Huang et al.,34 where they found the dissociation of two adsorbed H2O molecules on one O atom precovered Co(0001) surface [2H2O + O = 3OH + H]. Since the stepwise route for OH dissociation is found more favorable kinetically than the direct route for two coadsorbed OH + OH, and the migration barrier is very low (0.25 eV), we only computed O−H dissociation after the migration step, (n − 1)O + 2OH = nO + H + OH and nO + H + OH = (n + 1)O + 2H. For n = 2−8 (θO = 0.22−0.89 ML), the dissociation barrier of both steps is in the range of 0.9−1.2 eV. For n = 2−5 (θO = 0.22−0.56 ML), the dissociation reaction is exothermic, while it becomes endothermic by about 0.27 eV for n ≥ 6. For H2 desorption from the surface, (n + 1)O + 2H = (n + 1)O + H2(g), the desorption energies are in the range of 1.1− 1.2 eV for n = 2−5 (θO = 0.22−0.56 ML), and 0.49−0.50 eV for n = 6−7 (θO = 0.67−0.78 ML). For n = 8 (θO = 0.89 ML), H2 desorption is exothermic by 0.08 eV, indicating that the surface fully covered by oxygen atoms cannot adsorb H2. 3.5. High OH and O Coverage. The most stable adsorption configurations of OH species at different coverage are shown in Figure 6a. The stable configurations of OH species are grown with the trimer moiety, (OH)3, where one OH in the middle was tilted at the top site as H acceptor and the other two at the quasi-4-fold hollow sites as the H donor. It is very interesting to note that the average adsorption energies (−4.07 eV) are equal for the adsorption of (OH)3, (OH)6, and (OH)9 and that they are higher than that (−3.93 eV) of one

OH adsorption. In the small (OH)3 unit, the H-bonding distance is about 1.7 Å. Thermodynamically, the formation of (OH)3, (OH)6, and (OH)9, is exothermic by 2.28, 4.55, and 6.84, respectively, on the basis of gaseous H2O molecules [nH2O(g) = nOH) + nH2(g)/2]. From the above analysis, along with the potential energy surfaces (Figure 4), the formation of (OH) 3 , (OH) 6 , and (OH) 9 is possible thermodynamically and kinetically. The most stable adsorption configurations of O atoms at different coverages are computed. As shown in Figure 6b, all O atoms are at the quasi-4-fold hollow sites, and they form lines for n = 3, 6, and 9 (θ = 0.33, 0.67, and 1 ML, respectively). At 1 ML O coverage, the adsorbed O forms a perfect p(1 × 1)-O structure, and this result is in agreement with the results proposed by Müssig et al.18 The calculated average adsorption energies (−3.05, −3.07, and −3.08 eV, respectively) are nearly the same with the increase of the O coverage, and they are higher than that (−2.92 eV) of one O atom. Thermodynamically, the formation of 0.33, 0.67, and 1 ML O coverage is exothermic by 1.55, 3.26, and 4.98, respectively, on the basis of gaseous H2O molecules [nH2O(g) = nO + nH2(g)] or endothermic by 0.73, 1.29, and 1.87 eV, respectively, on the basis of OH coverage [nOH = nO + nH2(g)/2]. 3.6. Desorption of H2O and H2 on Fe(111). On the basis of the stable adsorption states, the effects of temperature under given pressure can be obtained by applying ab initio atomistic thermodynamics.35,36 Ab initio atomistic thermodynamics is a convenient tool to solve problems referring to reaction conditions.37−41 The detailed description of the procedure for water adsorption can be found in our previous work.15 On the basis of Gibbs free energy changes of the related reactions at different temperatures, one can estimate the reaction temperature. In our previous studies, we found excellent agreement between experiment and theory in desorption temperatures of H2O and H2 under UHV condition on the Fe(100)15 and Fe(110)16 surfaces. We therefore computed H2O and H2 desorption from the Fe(111) surface under the same pressure of 1.3 × 10−13 atm (or 1 × 10−10 Torr) for direct comparison 11720


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Fe(100) surfaces are more dense and have the most stable adsorption sites at close distances, and the Fe(111) surface is more open and has the most stable adsorption sites in remote places. For H2O adsorption, the order of the adsorption energy correlates with the order of surface stability, i.e., the most stable Fe(110) surface has the lowest adsorption energy (−0.38 eV), while the least stable Fe(111) surface has the largest adsorption energy (−0.58 eV). For the adsorption of H, O, and OH, however, the energetic data do not show such correlations. For one H atom, the order of the adsorption energy is Fe(110) > Fe(111) ≈ Fe(100), i.e., the most stable Fe(110) surface has the largest adsorption energy (−0.76 eV), while those of the Fe(100) and Fe(111) are lower (−0.45 and −0.46 eV). For one O atom, the adsorption energies (−3.43 eV) on the Fe(110) and Fe(100) surfaces are higher than that on the least stable Fe(111) surface (−2.92 eV). For the adsorption of OH group, the most stable Fe(110) surface has the largest adsorption energy (−4.28 eV), while the least stable Fe(111) surface has the lowest adsorption energy (−3.93 eV). Detailed analysis and comparison reveal that these adsorption energies correlate with the coordination numbers of the adsorbed species. For example, the adsorbed H atom has three Fe−H bonds on the Fe(110) surface and two Fe−H bonds on the Fe(100) and Fe(111) surfaces. The adsorbed O atom has four Fe−O bonds on the Fe(110) and Fe(100) surfaces and two Fe−O bonds on the Fe(111) surface. The adsorbed OH group has three Fe−O bonds on the Fe(110) surface and two Fe−O bonds on the Fe(100) and Fe(111) surfaces. This correlation is also found in the coadsorbed OH + H and O + 2H, i.e., the most stable Fe(110) surface has the largest coadsorption energy (−1.66 and −2.23 eV, respectively), while the least stable Fe(111) surface has the lowest coadsorption energy (−1.09 and −0.99 eV, respectively). For the dissociation of adsorbed water [H2O = OH + H], the first O−H dissociation barriers on three surfaces are close; however, large differences are found for the dissociation energies. For example, the most stable Fe(110) surface is most exothermic, and the least stable Fe(111) surface is least exothermic. It is also worth noting that the dissociation barrier on the Fe(110) surface is greater than the adsorption energy, while the dissociation barriers on the Fe(100) and Fe(111) surfaces are close to their adsorption energies. For OH dissociation [OH = O + H], the O−H dissociation barrier on these three surfaces is very close; however, the dissociation is exothermic on the Fe(110) and Fe(100) surface and slightly endothermic on the Fe(111) surface. In addition, differences are also found for the OH disproportionation reaction, [2OH = H2O + O]. For the possible equilibrium, H2O formation on the Fe(110) and Fe(100) surface has higher barriers and is strongly endothermic compared to that on the Fe(111) surface. The lowest barrier and reaction energy on the Fe(111) surface reveal good equilibrium, while on the Fe(110) and Fe(100) surfaces, surface OH groups are more dominant surface species. On the basis of the computed adsorption energies, it is also possible to compute the desorption temperature from ab initio atomistic thermodynamics under UHV conditions. On the Fe(110) and Fe(100) surfaces, which have close H2O adsorption energies (−0.38 and −0.41 eV, respectively), the computed H2O desorption temperatures are also close (232 and 240 K, respectively), and it is worth noting that these computed desorption temperatures are very close to the experimentally estimated values (225 and 220 K, respectively).

with the Fe(100) and Fe(110) surfaces and also for providing some valuable data for the experiment. As shown in Figure 7a, the computed H2O desorption temperature for the adsorbed H2O molecule [H2O(s) =

Figure 7. H2O and H2 desorption temperatures at 1.3 × 10−13 atmosphere on the Fe(111) surface (s for surface species; and g for gaseous species).

H2O(g)] is 278 K. Since the adsorption energy (0.58 eV) and the first dissociation barrier (0.54 eV) of H2O are of the same magnitude, the desorption temperature at 278 K should also represent H 2 O dissociation temperature to form the coadsorbed OH + H on the surface. The computed H2O desorption temperature from OH disproportionation [2OH(s) = O(s) + H2O(g)] is 375 K. On O-precovered surfaces, H2O desorption temperatures, which are in the range of 342−366 K, are higher than that on the clean surface (Figure 7b), and this is due to H-bonding stabilization between the surface O atom and the adsorbed H2O molecule. The computed H2 desorption temperature is 281 K on the clean surface [2H(s) = H2(g)], which is in agreement with the study of Wang et al.29 However, it is noted that at very high O coverage, the adsorption of gaseous H 2 is very weak and that H 2 adsorption is thermodynamically not possible at 1 ML O coverage.

4. COMPARISONS AMONG THE FE(100), FE(110), AND FE(111) SURFACES Table 1 lists the results of water dissociative adsorption on the Fe(110), Fe(100), and Fe(111) surfaces for direct comparison. Despite the qualitative similarities among the three surfaces, there are significant differences in several aspects. On the basis of the computed surface energies,42 the stability order is Fe(110) > Fe(100) > Fe(111). Structurally, the Fe(110) and 11721


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The Journal of Physical Chemistry C Table 1. Comparisons among Fe(110), Fe(100), and Fe(111) Surfacesa 110 H H2 O OH H2O OH + H O + 2H

100

Adsorption Energy (Eads) −0.76 −0.45 −1.52 (−1.69) −0.89 (−1.11) −3.43 −3.43 −4.28 −4.14 −0.38 (−0.62) −0.41 (−0.58) [−0.52b] −1.66 −1.29 [−1.25b] −2.23 −1.76 Water Dissociation Barrier and Energy (Ea/ΔER) H2O = OH + H 0.68/-1.28 0.41/−0.88 [0.68/−0.73b] {0.63/−0.79c} OH + H = O + 2H 0.90/−0.57 0.85 [0.91b]/−0.47 {0.68/−0.74c} OH + OH = O + H2O 0.78/0.38 0.74/0.56 H2O and H2 Desorption Energy and Temperature (Edes/T) H2O(s) = H2O(g) 0.38/232K [225 Kd] 0.41/240K [220 Ke] {0.43c} 2OH(s) = O(s) + H2O(g) 0.84/333K [315 Kd] 0.72/315 K [310 Ke] 2H(s) = H2(g) 1.52/411K [430−480Kf] 0.89/315K [300 Ke]

111 −0.46 −1.00 (−1.20) −2.92 −3.93 −0.58 (−0.74) [−0.85b] −1.09 [−1.76b] −0.99

0.54/−0.51 [0.58/−0.91b] 0.93 [1.46b]/0.10 0.12/0.10

0.58/278K 0.90/375K 1.00/281K

a

Energies in eV; and temperature in K. The dispersion corrected (DFT-D2) adsorption energies for H2O and H2 are given in parentheses, and the available literature data are given in square brackets. bHSE06 (ref 13). c(RPA+EX)@PBE (ref 14.). dUltraviolet photoelectron spectroscopy (ref 12). e Temperature-programmed desorption (ref 5). fThermal desorption spectroscopy (ref 44).

interaction at up to 1/4 ML coverage, while at up to 1 ML coverage, the average adsorption energy decreases steadily despite the H-bonding among the adsorbed OH groups. On the Fe(100) surface, the most stable adsorbed OH groups have Hbonding and form a well-ordered linear structure at the bridge site from low to high coverage, and the average OH adsorption energy increases with the increase of OH coverage. On the Fe(111) surface, the stable configurations of OH species have small (OH)3 moieties, and the average OH adsorption energy is independent of the coverage. On the Fe(110) surface, the maximum surface coverage of OH and O is 0.75 and 0.44 ML, respectively. Thermodynamically, the Fe(100) surface can have 1 ML OH and O coverage, although the formation of 1 ML O coverage is kinetically hindered. On the Fe(111) surface, the formation of 1 ML OH and O is thermodynamically and kinetically possible. Recently, Lazar et al., 13 computed monomeric H2O dissociative adsorption on the Fe(100) and Fe(111) surfaces by using the hybrid HSE06 method. On the basis of our systematic study, it is possible to compare the results between PBE and HSE06. On the Fe(100) and Fe(111) surfaces, HSE06 favors stronger H2O adsorption than PBE by 0.11 and 0.27 eV, respectively. For the closely coadsorbed OH + H, however, the adsorption energies at HSE06 and PBE are very close on the Fe(100) surface (1.25 and 1.29 eV, respectively), while a large difference is found on the Fe(111) surface (1.76 and 1.09 eV, respectively). For H2O dissociation, HSE06 favors a higher barrier than PBE on the Fe(100) and Fe(111) surfaces by 0.27 and 0.04 eV, respectively. In addition, HSE06 shows that the surface coadsorbed OH + H represents a deep minimum on the potential energy surface on the Fe(100) and Fe(111) surfaces, PBE shows that the deep minimum on the potential energy surface is the coadsorbed OH + H on the Fe(111) surface, while the coadsorbed O + 2H on the Fe(100)

Very good agreement is also found for water desorption from surface OH disproportionation [2OH = O + H2O(g)] on the Fe(110) and Fe(100) surfaces between theory (333 and 315 K, respectively) and experiment (315 and 310 K, respectively). In addition, the computed H2 desorption temperatures (411 and 315 K, respectively) on the Fe(110) and Fe(100) surfaces agree also reasonably with the available experimental data (430−480; and 300 K, respectively). These agreements validate our computational models and methods. Apart from these differences at the lowest coverage, large differences are also found at high coverage. On the surfaces, adsorption and H-bonding contribute jointly to the stability of adsorbed (H2O)n clusters, and the adsorption structures are different due to the different surfaces. On the Fe(110) surface, for n = 1−5, the stable adsorption configurations can be deduced by stepwise H2O adsorption, and both (H2O)4 and (H2O)5 have star-like structural patterns, while the stable configurations are based on a hexagonal moiety for n ≥ 6. On the Fe(100) surface, the preferred configuration is based either on four-membered rings or on five-membered rings, which are very close in energy. On the Fe(111) surface, both dispersed and aggregated structures are close in energy despite their structural differences at low coverage, while the aggregated configurations at high coverage are based either on fivemembered rings or on six-membered rings. It is worth noting that on the Fe(111) surface, H-bonding plays a more important role than adsorption at high coverage and that the H2O molecule, which has dominant H-bonding interaction instead of adsorption, has the second H−O bond pointing to the surface. Such structures have not been found on the Fe(110) and Fe(100) surfaces. There are significant differences in the adsorption of OH and O. On the Fe(110) surface, the OH groups are dispersedly and perpendicularly adsorbed and do not have H-bonding 11722


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endothermic for n ≥ 6 (θO ≥ 0.67 ML). (e) For H2 desorption from the surface, (n + 1)O + 2H = (n + 1)O + H2(g), the desorption energies are in the range of 0.8−1.2 eV for n = 1−5 (θO = 0.11−0.56 ML) and 0.49−0.50 eV for n = 6−7 (θO = 0.67−0.78 ML). For n = 8 (θO = 0.89 ML), H2 desorption is exothermic by 0.08 eV, indicating that the surface fully covered by oxygen atoms cannot adsorb H 2 . (f) The stable configurations of OH species are grown with trimer moiety (OH)3, where one OH is in the middle as H acceptor and the other two are as H donors. The average OH adsorption energies are equal and coverage independent. The formation of 1 ML OH is possible thermodynamically and kinetically. (g) For the adsorption of O atoms, at different coverages, all of the O atoms are at the quasi-4-fold hollow sites. The average O adsorption energies are equal and also are coverage independent. For θ = 0.33, 0.67, and 1 ML, there are parallel well-ordered lines, in agreement with the experiment, in particular at the 1 ML coverage. (h) Under ultrahigh vacuum conditions, the desorption temperature of H2O and H2 on the clean Fe(111) surface was computed using ab initio atomistic thermodynamics, and that of H2 agrees with the experimental result. On the nO-precovered Fe(111) surfaces, H 2 O desorption temperatures are higher than that on the clean surface, and this is because of the H-bonding interaction between surface O and H2O. (i) These results on the Fe(111) surface along with our previous investigations into water dissociative adsorption on the Fe(110) and Fe(100) surfaces at different coverages reveal their intrinsic differences in water dissociative adsorption and provide a basic understanding of the mechanisms of water interaction with metal surfaces.

surface. This reveals the agreement and difference between HSE06 and PBE. In addition, on the Fe(110) surface, the coadsorbed O + 2H represents the deep minimum on the potential energy surface at PBE. For studying water adsorption on iron surfaces, it is suggested to include the long-range dispersion correction for van der Waals (vdW) interaction because of the weak adsorption and H-bonding. On the basis of the semiempirical GGA-type functional (PBE-D2) proposed by Grimme,43 it is found that the dispersion corrections (DFT-D2) overestimate the adsorption energies of H2O and H2 on the iron surface compared with only PBE. For H2O adsorption, the overestimation on the Fe(110), Fe(100), and Fe(111) surfaces is 0.24, 0.17, and 0.16 eV, respectively. For H2 adsorption, the overestimation on the Fe(110), Fe(100), and Fe(111) surfaces is 0.17, 0.22, and 0.20 eV, respectively. Since PBE can reproduce reasonably the desorption energies of H2O and H2 on the Fe(110) and Fe(100) surfaces, PBE might be an applicable method for iron systems. Our results might provide a benchmark example for the further development and evaluation of the theoretical methodology for correctly accounting for such very weak but very important chemical forces in many systems since not all of these results are available from experimental studies.

5. CONCLUSIONS Systematic density functional theory calculations were performed to investigate the adsorption and dissociation of H2O on the clean and nO-precovered Fe(111) surfaces. The following conclusions can be outlined. (a) On the clean surface, the most stable adsorbed positions of the species H, O, and OH are all at the quasi-4-fold hollow site. For H2O molecule, the most stable adsorption configuration is located at the top site with the H2O molecule parallel to the iron surface. All of these species can migrate easily on the Fe(111) surface. (b) For the adsorption of (H2O)n clusters, adsorption and H-bonding play the role in stabilizing the cluster structures. At low coverage, the adsorbed H2O molecules can be dispersed or aggregated close in energies while only aggregating at high coverage. At high coverage, different aggregated adsorption configurations with five-membered and six-membered rings are close in energies. In the aggregated adsorption configurations, there are adsorbed H2O molecules tilted at the top sites and H-binding stabilized H2O molecules over the deep hollow sites with one H atom pointing to the surface. This shows that the adsorbed H2O molecules on the Fe(111) surface might not have defined structures, in agreement with the experimentally observed results. (c) Monomeric H2O dissociation is favored both thermodynamically and kinetically. Since there is no H-bonding interaction in the dispersed (H2O)2, there are no mutual effects in dissociation. In the aggregated (H2O)2 stabilized by Hbonding, however, the dissociation is less favorable kinetically. (d) On the nO-precovered surfaces (nO = 1−8, θO = 0.11−0.89 ML), the potential energy surfaces are similar. The H2O adsorption energy, [nO + H2O(g) = nO + H2O(s)], is in the range of 0.86−0.99 eV and is coverage independent. The first dissociation step, nO + H2O = (n − 1)O + 2OH, has a very low barrier and reaction energy, revealing its high reversibility and fair equilibrium in nature. For OH successive dissociation, (n − 1)O + 2OH = nO + H + OH as well as nO + H + OH = (n + 1)O + 2H, the dissociation barriers are in the range of 0.9−1.2 eV and are coverage independent. For nO = 1−5 (θO = 0.11− 0.56 ML), the dissociation reaction is exothermic, while it is

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ASSOCIATED CONTENT

S Supporting Information *

Model test for the Fe(111) surface on H2O adsorption; adsorption energies and structure parameters of the adsorbed species on the Fe(111) surface; adsorption energies and structure parameters of (H2O)n clusters on Fe(111) surface; computed structure parameters of the H2O molecule in (H2O)n clusters adsorbed on dh sites with H atom points to the surface; adsorption energies and structure parameters of the IS, TS, and FS for H2O direct dissociation and 2H2O dissociation on Fe(111) surface; adsorption energies and bond distances of IS, TS, and FS for H2O dissociations on the nO-precovered Fe(111) surface; adsorption energies and desorption temperature of H2O and H2 at 1.3 × 10−13 atmosphere; optimized geometries for the stationary points of 2H2O dissociation; optimized geometries for the stationary points of H2O dissociation on the nO (n = 1−8) precovered surface on the Fe(111) surface.The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b02297.

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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 (no. 21273262&21273266), and 11723


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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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Coverage Dependent Water Dissociative Adsorption on the Clean and

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