Dissociation of Water at Iron Surfaces: Generalized Gradient

Nov 14, 2012 - Water molecule prefers oxygen-down adsorption onto the top site. The geometry is displayed in Figure 1. The preference of the top site ...
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Dissociation of Water at Iron Surfaces: Generalized Gradient Functional and Range-Separated Hybrid Functional Study Petr Lazar and Michal Otyepka* Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, tr. 17. listopadu 12, 771 46 Olomouc, Czech Republic S Supporting Information *

ABSTRACT: Interaction of water with iron surface is involved in many significant processes like corrosion and water treatment by zerovalent iron nanoparticles (nZVI). We used a density functional theory to study adsorption and chemical reaction of a single water molecule with two low-index surfaces of iron, Fe(100) and Fe(111). We used generalized gradient form (PW91) of the density functional and also range-separated hybrid functional (HSE06), which incorporates a fraction of the Hartree−Fock exchange. A water molecule adsorbs on both surfaces with oxygen atom pointing on top a Fe atom and has higher affinity to the Fe(111) surface. The adsorbed water molecule can dissociate into H + OH (H−Fe−OH) species attached to the Fe surface with an activation barrier of 15.7 and 13.3 kcal/mol for the (100) and (111) surface, respectively. The hybrid functional yields similar energies for adsorption but predicts higher dissociation barriers compared to the generalized gradient functional. The HSE06 calculation reveals that H−Fe−OH is a deep minimum on the reaction profile of the studied process, in particular on the Fe(111) surface. This indicates that dissociated species can play an important role in reactivity of nZVI with pollutants in water treatment. The HSE06 functional also improves the overall agreement between theoretical calculations and previous experimental studies of the adsorption of water on iron surfaces.

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later revised in DFT study by Jung et al.,9 who employed GGA functional as well but used large supercell modeling Fe surface and found locally stable state of H2O on the Fe(100) surface with an activation barrier of 8 kcal/mol for a dissociation into OH + H. In our recent quantum-chemical study10 we showed that for the H2O molecule interacting with the Fe atom the gas phase activation energy for H2O → OH + H reaction is actually much higher, 23.7 kcal/mol. This value was obtained using the coupled cluster method including single and double electron excitations and perturbative triple electron excitations, CCSD(T). This method provides very accurate and physically consistent treatment of the energetics stemming from electron many-body effects (i.e., electron exchange and correlation). We have also demonstrated large differences in reaction energetics (in particular reaction barriers) among various density functionals. So one may naturally ask whether such differences in calculated barrier heights arise from differences between an electron structure of the iron surface and the iron atom or from insufficient description of a molecule−surface system within the generalized gradient approximation for the exchange-correlation potential. Therefore, we present a DFT study of the dissociation of the water molecule on the Fe surface using orbital-dependent

INTRODUCTION A reaction of water with iron surfaces is one of the fundamental processes in nature. Water-induced changes of iron surfaces are important for electrochemistry, catalysis, and steel industry (corrosion). The interest in the reaction has been recently greatly increased because zerovalent iron and in particular its nanoparticles are becoming widely used in reductive technologies for ground and wastewater decontamination.1−4 The pollutants in contaminated water are, after the reaction with iron nanoparticles, left anchored in mineral surrounding in less toxic or nontoxic phases, and products of iron oxidation (Fe3O4, Fe2O3, FeOOH) are completely nontoxic. However, the detailed mechanism of dissociation of pollutants, the role of iron nanoparticles, their size, and interplay with water molecules is still unclear. Surprisingly, consistent theoretical understanding of thermodynamics of water dissociation on Fe surfaces is still missing. The standard method of choice for fully quantum-mechanical calculations of a molecule−surface system, which is relatively large-scale problem, is the density functional theory (DFT).5,6 Eder et al.7 used common DFT approach with generalized gradient approximation (GGA) for exchange-correlation potential and obtained negligible barriers for the dissociation of H2O on Fe(100) and Fe(110), which would mean that the water molecule dissociates into OH and H species spontaneously. Such a conclusion contradicts experimental observation of stable water species on the Fe(100) surface8 and was © 2012 American Chemical Society

Received: October 3, 2012 Revised: November 13, 2012 Published: November 14, 2012 25470

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hybrid functional HSE0611 besides common semilocal GGA density functional in the parametrization by Perdew and Wang.12 Hybrid functionals replace certain fraction of the GGA exchange with the exact Hartree−Fock (HF) exchange and have proven to provide an improved description of molecular properties such as vibrational spectra, formation enthalpies, and reaction barriers (see refs 13−16 and references therein). Because hybrid functionals employ a nonlocal operator for the exchange energy, they can also partly correct the errors stemming from the missing derivative discontinuity16 and reduce self-interaction error in present local functionals. HSE06 functional is a range-separated hybrid functional, in which the exchange term is partitioned into a short- and a long-range part. The GGA exchange is mixed with the HF exchange only in the short-range part, whereas the long-range part is still treated by semilocal GGA functional. This construction reduces computational costs because proper evaluation of the long-range component of the HF exchange would require very dense kpoint sampling of the Brillouin zone. HSE06 also obeys homogeneous electron gas limit and, consequently, provides more consistent results for extended systems, in particular metals, than another popular hybrid functional B3LYP.17,18 We studied two low-index surfaces of iron, Fe(100) and Fe(111), in order to examine the influence of surface geometry. Both surfaces are rather open in a bcc structure and may therefore imitate the surface of iron nanoparticles better than more densely packed surfaces of iron. The 3-fold symmetry of the (111) surface also mimics the symmetry of facets commonly found in nanoparticles. We determined the stable positions of the H2O, OH molecules, and the O, H atoms at the surface and employed the climbing image nudged elastic band method (CINEB) to calculate reaction barriers. For the geometries associated with important reaction steps, we calculated singlepoint energies using the HSE06 hybrid functional.

slab. The energy barriers blocking the dissociation were determined throughout the CI-NEB method.23 The CI-NEB method utilizes several intermediate structures (images) to map a reaction path and is constructed in a way which makes one of images climb up the reaction path and reach the saddle point. We utilized four intermediate images; lower number of images gives the CI-NEB algorithm higher flexibility, needed to converge to the minimum-energy path. We used the smearing method of Methfessel−Paxton24 for the calculations based on forces (geometry relaxation, CI-NEB). Then, we did one static calculation using tetrahedron smearing with Blochl corrections25 to get accurate total energies. The hybrid function calculations were performed on the geometries and the wave functions preconverged using the GGA functional. We note that HSE06 calculations are computationally much more demanding that a GGA calculation, so the full geometrical optimization using hybrid functional would not be feasible. This approach was recently justified by Cohen et al.,16 who showed that GGA- and HSE06-based geometries are similar, and when comparing various exchange-correlation functionals, the total energies of important stationary reaction points are less affected by small geometrical differences then by the form of the exchange-correlation functional itself. In following, the values of the energy without functional declaration refer to the calculation using PW91 functional; HSE06 energies were calculated only for selected geometries, and their values are explicitly declared in the text.

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RESULTS Reaction on the Fe(100) Surface. The calculation of reaction barriers using the CI-NEB method necessitates first the determination of the stable adsorption states of relevant species. Thus, we first calculated the most stable positions at the surface for the H2O, OH molecules and the H, O atoms. One finds three high symmetry sites on the Fe(100) surface: the on-top (t) site above one the Fe atoms, the 2-fold bridge (b) site connecting two neighboring Fe atoms, and the 4-fold hollow (h) site. The positions of sites are sketched in Figure 1. We

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COMPUTATIONAL DETAILS The Vienna ab initio Simulation Package (VASP)19,20 was applied for the DFT calculations. The projector augmented wave (PAW) construction was used to describe the interaction between ions and valence electrons. Atomic forces were relaxed within the conjugate-gradient algorithm wherever structural relaxations were required. The convergency of the total energy with respect to computational parameters such as the number of k points for the Brillouin zone integration was tested. Large energy cutoff of 400 eV was used to ensure sufficiently accurate total energies and forces. The structural relaxation was stopped when the forces acting on atoms were converged to 10−3 eV/Å. The spin polarization was allowed in all calculations. Fe crystallizes in the body-centered cubic structure, which was considered for all studies. The surface was modeled by a periodic slab with vacuum region separating periodically repeated slabs. Our previous slab calculations showed21,22 that the slabs six layers thick are sufficiently large to get converged surface energies and consequently ensure also proper modeling of the adsorption energies. The (3 × 3) and the (2 × 2) surface structure were used as the surface geometry of the Fe(100) surface and the Fe(111) surface, respectively. These geometries resulted in the slabs, which consisted of 54 atoms for both surfaces. The vacuum spacing of repeated cells was 14 Å. A gamma-centered 3 × 3 × 1 k-point mesh was used for both surface slabs. The adsorption energies are calculated as the total energy of the Fe slab with adsorbed species relative to the sum of the energies of isolated H2O molecule and isolated

Figure 1. The precursor state of H2O molecule adsorbed on the Fe(100) surface and high symmetry adsorption sites: the on-top (t) site above Fe atom, the 2-fold bridge (b) site connecting two neighboring Fe atoms, and the 4-fold hollow (h) position.

situated an atom, or a molecule into respective position above the surface slab, and let the distance of adsorbed molecule and its geometry to relax. We allowed also the relaxation of iron atoms in the slab. Table 1 shows calculated adsorption energies and distances. Water molecule prefers oxygen-down adsorption onto the top site. The geometry is displayed in Figure 1. The preference of 25471

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hollow site and vary the positions of the hydroxyl group. The results in Table 1 suggest that OH slightly prefers the bridge site over the top position. The preference of the electron-rich bridge site was observed also experimentally.8 We found a shallow local minimum of the energy at the top position, contrary to the DFT study by Jung et al.9 claiming that the top site is locally unstable. Adsorbed OH has the oxygen-down configuration, and the OH bond is tilted by 70° outward the surface plane. The tilted configuration is also in agreement with experimental observations.8 The O−H bond length of 0.97 Å is very close to the bond length of an isolated hydroxyl group (0.99 Å). The adsorption energy given by GGA is −27.4 kcal/ mol for OH(b) + H(h) relative to the sum of the energies of isolated H2O molecule and Fe slab. The adsorption energy agrees well with the value of −26.1 kcal/mol calculated by Eder et al.7 The hybrid functional again yields similar energy; HSE06 adsorption energy for the bridge site is −28.9 kcal/mol. Now we can consider the dissociation of the H2O molecule at the preferred top position. The OH group prefers to sit at the bridge site, so the dissociation process must involve a transition from the top to the bridge position. Notice that the dissociation would be exothermic for both top and bridge positions of OH molecule because in either site the energy of OH is more negative than the energy of adsorbed water molecule. On the basis of these facts, we considered two possible reaction paths:

Table 1. Adsorption Energy E and the O−Fe Distance d to the Averaged Fe Surface Layer on the Fe(100) Surfacea (t) H2O E (kcal/mol) d (Å) OH + H(h) E (kcal/mol) d (Å) a

(b)

(h)

−11.0 (−12.0) 2.22

−8.0 (−8.4) 2.11

−4.3 3.19

−19.4 1.82

−27.4 (−28.9) 1.45

−27.3 1.27

The HSE06 energies (when available) are values in parentheses.

the top site is in accordance with the H2O adsorption pattern found on the most of metal surfaces (e.g., Pd, Pt, Al, Cu, Ag, Au).26 The oxygen atom does not lie exactly above surface Fe atom but is slightly shifted toward the bridge site, reflecting partial asymmetry of the H2O molecule. The O−H bonds are not oriented parallel to the surface but are tilted by 40° outward. The Fe−O distance of 2.22 Å is in agreement with 2.29 Å reported in previous DFT study.9 The nearest-neighbor Fe atom is exerted out of the (100) plane by 0.1 Å due to the interaction with the molecule. If water molecule rotates along the z-axis (perpendicular to the surface), the adsorption energy changes only negligibly. The tilted configuration and Fe−O distance agree remarkably with the geometry obtained for H2O molecule bound to Fe atom only,10 indicating that the interaction of water molecule with sole Fe atom may capture some aspects of the chemistry found on the iron surface. The adsorption energy at the top site is −11.0 kcal/mol in GGA calculation. The hybrid functional yields quite similar value of the adsorption energy, −12.0 kcal/mol. The bonding to the surface is realized by hybridization between the oxygen lone pair of electrons in water molecule and the Fe d electrons. The results in Table 1 reveal considerable differences among symmetric adsorption positions, with (b) and (h) sites having higher adsorption energies than the (t) position. This finding is in contrast to a previous DFT study by Eder et al.,7 which found similar energies for high symmetry sites. On the other hand, the adsorption energy and the energetic difference of 3.0 kcal/mol between top and bridge site agree very well with the results of Jung et al.9 Jung and co-workers also demonstrated that insufficiently large (2 × 2) slab employed in ref 7 is the reason for disagreement in the adsorption energetics. Futher justification comes from the HSE06 calculation, which predicts that the difference between adsorption at the top and bridge site is 3.6 kcal/mol. Adsorption energies are thus very similar in local generalized gradient and in nonlocal hybrid functional. Notice also that the adsorption of water is exothermic on all high symmetry sites, even on the hollow site. In general, the adsorption energies, and relatively small energetic differences between them, indirectly indicate that the H2O molecule may diffuse on the Fe(100) surface already at low temperatures, and consequently, the results corroborate the idea that water dissociates via a mobile precursor on the Fe(100) surface.27,28 Next, we have to find the positions of the OH and H species. Adsorption of hydrogen on the Fe(100) surface was already studied theoretically7,9,29 and also experimentally by means of electron-energy loss vibrational spectroscopy.30 The studies concluded that hydrogen first adsorbs in the 4-fold hollow sites and converts to bridge sites with increasing coverage. Theoretical studies also revealed low diffusion barriers and high mobility of H on the Fe surface. Thus, we can assume that hydrogen will reach the hollow site; we put the H atom into the

H 2O(t) → OH(t) + H(h) dissociation followed by OH(t) → OH(b) transition

(1)

H 2O(t) → H 2O(b) + transition followed by H 2O(b) + H(h) dissociation

(2)

These two processes are not equivalent because the adsorption energy difference between top and bridge is 3.2 and 8.0 kcal/ mol for the H2O and OH molecule, respectively. The CI-NEB calculation reveals that the reaction path (2) has lower dissociation barrier of 7.4 kcal/mol, whereas the barrier associated with the path (1) is 20.8 kcal/mol. Reaction 2 proceeds first by slight rotation of the H2O molecule at the bridge site, which breaks initial symmetry of the system and brings one of H atoms nearer to the hollow site. In the following steps the O−H bond elongates, hydrogen approaches the hollow site, until the O−H bond is broken, and hydrogen forms a Fe−H bond with the surface. This picture is in excellent agreement with the results of the study of Jung et al.9 The calculation with the hybrid functional yields interesting differences; while the adsorption energies in the top and the bridge site defining the first step in the dissociation process (2) are essentially unchanged, the energy of the transition state gets much higher in the HSE06 calculation. Consequently, the barrier calculated by the hybrid functional is 15.7 kcal/mol, twice as high as the barrier calculated by the PW91 form of GGA functional. We also estimated the influence of the energy of zero-point vibrations on initial reaction steps. We calculated vibrational frequencies of the gas-phase H2O molecule and then the H2O molecule in the initial adsorbed state and the transition state (before OH + H dissociation). The Fe atoms in the slab were frozen to avoid tedious calculation of the full Fe phonon spectrum. Respective zero-point energies (ZPE) and the frequencies of normal vibrational modes of H2O (symmetric 25472

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hollow site as the hydroxyl group dissociates. We assume that oxygen stays initially at the bridge site and the crucial step is the O−H bond breaking, which occurs when hydrogen approaches neighboring hollow site. The CI-NEB calculation with the generalized gradient functional gives the dissociation barrier of 8.9 kcal/mol for this process. The OH molecule first rotates out of symmetric position so that the O−H bond lies in direction of neighboring hollow site. In following images the hydrogen approaches the hollow site, breaks the O−H bond, and forms a new bond with the surface. Finally, the oxygen atom slides almost barrierless from the bridge site into the hollow position, and the water molecule is completely dissociated. The OH dissociation barrier of 8.9 kcal/mol is in contrast to the barrier of 18.2 kcal/mol calculated by Jung et al.9 The difference arises from a different choice of the dissociation path; Jung et al. used also the OH at the bridge site as a starting configuration but assumed that O and H approach neighboring hollow sites simultaneously, i.e., used different final configuration in the CI-NEB calculation. The ability of the CI-NEB method to find the proper (the lowest) transition state of course depends on the choice of initial and final states. Surprisingly, their barrier corresponds to temperatures in the range 265−306 K in Arrhenius type analysis, which compares well to the dissociation at 310 K observed in experiment.8 That may seem to contradict our result; however, the hybrid functional again increases the energy of the transition state, which results in the OH dissociation barrier of 21.1 kcal/mol. This barrier is in accord with experimental observations for the Fe(100) surface.8 The whole reaction is displayed in Figure 2 (the geometries of important configurations along the reaction path are displayed in the Supporting Information). Surprisingly, both functionals yield final reaction products differently. The PW91 functional predicts that hydrogen atoms prefer to stay in the Fe

stretch, asymmetric stretch, and bend mode) are listed in Table 2. The breakdown of normal modes in the transition state leads Table 2. Zero-Point Energies (in kcal/mol) and Frequencies (in cm−1) Calculated by PW91 Functional of Vibrational Modes of H2O (Symmetric Stretch, Asymmetric Stretch, and Bend Mode) for Initial Adsorbed State and the Transition State of the H2O Molecule on the (100) Surfacea sym stretch asym stretch bend ZPE

gas-phase H2O

H2O...Fe(100)

transition state

3654 (3657) 3759 (3756) 1590 (1595) 13.4

3550 3650 1566 14.3

1859 3607 1045 11.9

a The numbers in parentheses are experimental data31 for gas-phase H2O. The frequencies in the transition state do not correspond exactly to above mentioned modes because the symmetry of the H2O molecule is broken.

to slight lowering of the reaction barrier; nevertheless, in general the energy of zero-point vibrations has rather minor effect. We thus did not consider ZPE in following calculations. Following step in the reaction is the dissociation of the hydroxyl group, which resides at the bridge site as the product of preceding dissociation of water. The final state, adsorbed oxygen, was inspected in previous DFT studies.7,32 The 4-fold hollow position is the stable adsorption site of O on the Fe(100) surface. The 2-fold bridge site and the top site have the adsorption energy higher by 12.9 and 35.2 kcal/mol, respectively,7 and the adsorption of atomic oxygen remains a nonactivated process up to full monolayer coverage.32 High energy differences among adsorption sites indicate lower mobility of oxygen compared to hydrogen. These results suggest that oxygen has to move from the bridge site to the

Figure 2. Dissociation of water molecule on the Fe(100) surface calculated PW91 density functional (black curve, dots) and HSE06 hybrid functional (red curve, dashed lines). The arrows show the heights of energy barriers along the reaction path. 25473

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however corresponded to higher coverages (2/3, 1, and 2 ML) of the Fe(111) surface. We calculated the site preference of single hydrogen atom and found that it prefers the (sh) site over (dh) site by 4 kcal/mol. The top and bridge positions were unstable and hydrogen atom relaxed into the (sh) site. The adsorption energies are summarized in Table 3. The results demonstrate that the H2O molecule adheres stronger to

lattice rather than to form H2 molecules and desorb from the surface. Thus, the reaction products would be the oxygen and the hydrogen atoms sitting at hollow sites on the surface. On the other hand, HSE06 functional finds the lowest energy for the state with adsorbed oxygen and free H2 molecule. Such reaction products agree with experimental observation of H2 gas escaping nanosized Fe particles during anaerobic corrosion rates in water.33,34 Considering the barriers and relative stability of various dissociated species, we conclude that the HSE06, when compared to the GGA functional, makes the dissociation of OH less likely and consequently broadens its stability range on the surface. Of course, the scenario may change after further water molecules approach the Fe surface. Previous studies have investigated oxygen adsorption on the iron surfaces (for details, see ref 32 and references therein). On Fe(100) oxygen adsorption leads to full monolayer coverage of atomic oxygen, preceded by the formation of (2 × 2) superstructures at lower coverage. It can be expected that the oxygen atoms will replace hydrogen atoms and make hydrogen desorption out of Fe surface more likely. Completion of a full monolayer is followed by incorporation of additional oxygen atoms into subsurface octahedral positions, resulting in surface fcc-like O/Fe film, which passivates the surface against further oxidation. Reaction on the Fe(111) Surface. The (111) surface of bcc metals has a surface atomic arrangement exhibiting 3-fold symmetry and is a very open surface with atoms in both the second and third layers clearly visible when the surface is viewed from above. There are four high symmetry sites on the Fe(111) surface: the top site (t), the shallow hollow site (sh), the deep hollow site (dh), and the quasi-4-fold hollow (qh) site, which can be viewed also as the surface−subsurface layer bridge site. Figure 3 shows a view normal to the Fe(111) surface and

Table 3. Adsorption Energy E and the Normal Fe−O Distance d to Averaged Fe Surface Layer on the Fe(111) Surfacea H2O E (kcal/mol) d (Å) OH + H(sh) E (kcal/mol) d (Å)

(t)

(sh)

(dh)

(qh)

−19.2 (−19.5) 2.19

−18.9 2.10

−6.6 2.38

−8.7 2.37

−24.9 1.99

−5.8 1.66

−3.6 1.06

−35.0 (−40.5) 1.10

a

The symbols denote high symmetry sites on the surface; the top site (t), the shallow hollow site (sh), the deep hollow site (dh), and quasi4-fold hollow site (qh). The HSE06 energies (when available) are values in parentheses.

the (111) surface than to Fe(100) because the binding energy of 19.2 kcal/mol in preferred top site is higher than its corresponding value on the Fe(100) surface, 11.0 kcal/mol. The molecule again relaxes into the oxygen-down geometry with the H atoms tilted out of parallel orientation. The distance of the oxygen atom to averaged Fe surface layer is 2.19 Å. Underlying Fe atom is exerted out of the (111) plane by 0.07 Å due to the presence of the water molecule, and the Fe−O bond length is thus 2.12 Å. Table 3 displays also the adsorption properties of the hydroxyl group. The adsorption energies refer to the situation in which dissociated hydrogen atom is placed in preferred (sh) site. We found that (qh) site is the most stable adsorption site for OH, followed by (t) site with the energy difference of 10.1 kcal/mol. The molecule is slightly distorted from the ideal symmetric position and the distance to averaged Fe surface layer is 1.10 Å. The configuration with the OH molecule in the top position is still endothermic with respect to the energy of adsorbed H2O molecule and may therefore serve as an intermediate step in the dissociation, in analogy to the reaction path found on the Fe(100) surface. It is worth of notice that the energy difference between H2O and OH + H states on the Fe(111) surface, −15.8 kcal/mol, is very similar to −16.0 kcal/ mol obtained for the same species on the Fe(100) surface. Now we consider the dissociation of adsorbed H2O molecule into the OH and H species. We again assume that (sh) site is the final state of the H atom. The reaction path calculated by the CI-NEB method is similar to that found on the (100) surface; the O−H bond first elongates as H approaches neighboring (sh) site, until the bond is broken (the transition state), and new Fe−H bond is formed. The OH group, which stayed in (t) site during reaction, then almost barrierless diffuses into (sh) site. The CI-NEB calculation yields the dissociation barrier of 4.8 kcal/mol. The barrier is lower than that encountered at the Fe(100) surface, in compliance with expected higher catalytic activity of more open and corrugate (111) surface. Spencer, Schoonmaker, and Somorjai36,37 used iron catalysts for ammonia synthesis and demonstrated that for Fe single crystal planes the (111)/(100)/(110) reactivity ratio

Figure 3. The precursor state of H2O molecule adsorbed on the Fe(111) surface and schematic diagram of top view of the Fe(111) surface with the positions of symmetric adsorption sites. The high symmetry sites are the top site (t), the shallow hollow site (sh), the deep hollow site (dh), and the (qh) quasi-4-fold hollow site.

displays the positions of symmetric adsorption sites. In analogy with the Fe(100) surface, we calculated the most stable positions of the H2O molecule, the OH molecule, and the O atom. The stable positions for hydrogen were inspected in earlier DFT study; it was found that the most favored adsorption site for H is the (qh) site.35 The calculations 25474

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Figure 4. Dissociation of water molecule on the Fe(111) surface calculated PW91 density functional (black curve) or HSE06 hybrid functional (red curve). The arrows show the heights of energy barriers along the reaction path.

the water molecule in the precursor state at the Fe(100) surface. The local functional gives the dissociation barrier lower than the adsorption energy, and consequently, one should expect that the dissociation starts at lower temperatures than the desorption of water from the surface. On the contrary, the hybrid gives the energy of the transition state higher than the energy needed to desorb the molecule, i.e., predicts that the desorption should precede the dissociation. Hung, Schwartz, and Bernasek8 performed thorough experimental study of the oxidation of Fe(100) by water adsorption. Their temperature-programmed desorption and electron energy loss spectroscopy data suggest that the Fe(100) surface interacts sequentially with water, forming hydrogenbonded molecular clusters at low temperature (100 K) and low coverage. As the surface is warmed, first wetting occurs as the clusters break apart in the region of 150−170 K. Molecular water starts to desorb from the surface at 220 K. Interestingly, the dissociation of water into adsorbed hydroxyl and hydrogen starts after some of water molecules are desorbed from the surface. This picture is consistent with the results provided by HSE06 calculation (see above). The dissociation is complete at the temperature of 250 K, forming a p(1 × 2)-OH overlayer, with the O−H bond tilted from the surface normal. The hydroxyl overlayer disproportionates at even higher temperature, in accord with higher dissociation barrier of OH calculated by the HSE06 functional. Oxygen remaining on the surface following this process stays bound in the 4-fold hollow site.8 Although there is no detailed study of the reaction of water on Fe(111), Jiang et al. studied some aspects of the interaction of water with the clean and gallium precovered Fe(111) surfaces by thermal energy atom scattering (TEAS) and Auger spectroscopy.39 The results revealed linear increase in the oxygen Auger signal at 183 K, which is characteristic for the adsorption through a precursor state. Linear increase also indicates that water desorption was limited in this temperature range, which indirectly justifies our result that water sticks well (the adsorption energy is −19.5 kcal/mol in the HSE06 calculation) to corrugate Fe(111) surface. Furthermore, a rise

is 418:25:1. This result was corroborated by recent study of Fe nanoparticles, which revealed that the electrocatalytic activity of cubic Fe nanoparticles with the (100) surface structure is much higher than the activity of the nanoparticles enclosed with close-packed (110) facets.38 The hydroxyl group, residing at the (qh) site, can further dissociate into the final state, adsorbed oxygen, in analogy to the Fe(100) surface. We calculated the stable positions of the oxygen atom and found that the (qh) site is the most favorable position on the (111) surface. Notice that the 4-fold hollow position is the stable adsorption site of O also on the Fe(100) surface. Thus, the dissociation may proceed simply by removing hydrogen into neighboring (sh) site, while oxygen stays in the (qh) position. The barrier for this dissociation of OH is 19.9 kcal/mol according to the CI-NEB calculation. The whole reaction on the Fe(111) surface is displayed in Figure 4. The results demonstrate that the rate-limiting step is the breaking of the O−H bond in hydroxyl molecule because a rather high energy barrier has to be overcome. The OH molecule should experience the dissociation barrier of 19.9 kcal/mol according to GGA functional and even higher barrier of 33.6 kcal/mol according to HSE06 functional. The hybrid functional estimated higher reaction barriers, similarly to the Fe(100) surface. The barrier for the dissociation of H2O is only 4.8 kcal/mol using GGA functional but increases to 13.3 kcal/ mol when HSE06 is employed. The results reveal interesting differences among both surfaces; at the (111) surface, water molecules dissociate more easily, but the products, hydroxyl groups, face much high barrier blocking their further dissociation. Consequently, the OH groups are stable at the Fe(111) in broader range of temperatures. Iron nanoparticles have open and corrugate surfaces, similar to the Fe(111). The pollutants in contaminated water may indeed react with OH groups at the surface of a nanoparticle.

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DISCUSSION The hybrid functional HSE06 brought substantial and interesting changes to the reaction energetics, when compared to the GGA (PW91) calculation. The essential change concerns 25475

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both methods in hybrid functional a reasonable compromise. Still, the comparison of various functionals with the CCSD(T) result for the barrier height (associated with breaking of the O− H bond in water reacting with the Fe atom) in ref 10 shows that hybrid functionals underestimate the barrier and thus that higher-order correlation effects (CCSD(T) employs single, double, and iteratively also triple electron excitations) play also an important role.

in the oxygen signal at temperatures around 375 K was associated with the OH dissociation and hydrogen desorption (the details of the interaction of water with the surface could not be derived from TEAS measurement). High temperature needed to dissociate OH corresponds to the high barrier of 33.6 kcal/mol calculated by HSE06. The calculated energies and measured temperatures of respective processes are summarized in Table 4. The reaction

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CONCLUSIONS We performed DFT study of the adsorption and the dissociation of the water molecule at the (100) and (111) surfaces of iron. We found that the reaction proceeds via the molecular precursor state on both surfaces. The water molecule in the precursor state can decompose into the OH and H species. OH experiences further dissociation at elevated temperatures, resulting ultimately in oxygen and hydrogen atoms residing in 4-fold hollow sites. The calculation with the HSE06 hybrid functional reveals that H−Fe−OH is deep minimum on the reaction profile of the studied process, in particular on the Fe(111) surface. This indicates that the OH species on a surface can play an important role in reactivity of iron nanoparticles with pollutants in wastewater treatment.2−4 The initial dissociation barrier is lower at the (111) surface, in agreement with expected higher catalytic activity of open and corrugate (111) surfaces. The hybrid functional, when compared to semilocal GGA functional, yields very similar energies for the adsorption but much higher dissociation barriers. Inclusion of the exact HF exchange suppresses the tendency of local functional to favor delocalized charge distributions typical for transition states. The reaction energetics provided by the HSE06 functional is indeed in excellent qualitative agreement with the adsorption picture suggested in experimental studies.8,39 Nevertheless, one should keep in mind that a bias is hidden in the ratio of the HF exchange to the GGA exchange and that correct nonlocal treatment of higher-order correlation effects may be necessary to obtain accurate and consistent reaction energetics of a system such as a molecule on the transition-metal surface.

Table 4. Temperatures (in K) of Experimentally Determined Steps in H2O Decomposition on the Fe Surface and Respective Energies (in kcal/mol) Calculated by the HSE06 Functionala expt Fe(100) H2O(ad) → H2O(g) H2O(ad) → OH(ad) + H(ad) OH(ad)→ O(ad) + H(ad) Fe(111) OH(ad) → O(ad) + H(ad)

HSE06

220 243 310

12.0 15.7 21.1

≈375

33.6

a

The experimental data are from ref 8 for the (100) Fe surface and from ref 39 for the (111) surface. Subscript “ad” stands for adsorbed species and “g” for a gas phase.

energetics given by the hybrid functional agrees well with the experimental evidence. Comparing hybrid and local functional, experimental results corroborate the results of hybrid functional, in particular that the desorption of water should start at lower temperatures than the dissociation. It should be noted that the comparison with experiments should serve rather as a qualitative guideline since coverages are different. Further findings emerge when one compares the results with the study of water molecule interacting with Fe atom.10 The difference occurs already for the initial state, the water molecule adsorbed on the surface/atom. Our GGA calculation yielded rather strong bonding of H2O on the Fe(100) and Fe(111) surface, −11.0 and −19.2 kcal/mol, respectively. The HSE06 functional yielded very similar adsorption energies, −12.0 and −19.5 kcal/mol. However, the CCSD(T) calculation revealed that the electrons of the iron atom have negligible overlap with those of the water molecule, resulting in the interaction energy of −1.9 kcal/mol, although the equilibrium Fe−O distances and the geometries (O on top of Fe atom and tilted H atoms) are very similar. The study also showed that the most of functionals overestimate the adsorption energy (compared to the CCSD(T) calculation), even if the same geometry was used,10 which justifies the tendency of gradient-corrected functionals to overestimate adsorption energies on metal surfaces.40 Our results indicate that HSE06 hybrid functional does not improve adsorption energetics in this respect. Actually, the hybrid functional gives a slight increase of the adsorption energy compared to the GGA functional. This trend has been already observed in a systematic study of CO adsorption on Cu, Rh, and Pt surfaces.41 Although adsorption energies are not improved, the hybrid functional on the other hand gives significantly higher energies of all transition states and, consequently, increased the reaction barriers. Semilocal functionals such as GGA underestimate reaction barriers because they tend to give too low energies for delocalized charge distributions, which are typical for the transition states.16 On the contrary, the HF method overrates the energies of localized charges, which makes a combination of

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2 showing geometries of important configurations of water along the reaction path on the (100) and the (111) Fe surface, respectively; transition states determined by the CI-NEB method (see main text for details). This material is available free of charge via the Internet at http://pubs.acs.org.

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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 The authors gratefully acknowledge the support by the Operational Program Research and Development for Innovations−European Regional Development Fund (project CZ.1.05/2.1.00/03.0058 of the Ministry of Education, Youth and Sports of the Czech Republic), the Operational Program Education for Competitiveness−European Social Fund (project 25476

dx.doi.org/10.1021/jp3097814 | J. Phys. Chem. C 2012, 116, 25470−25477

The Journal of Physical Chemistry C

Article

(37) Spencer, N. D.; Schoonmaker, R. C.; Somorjai, G. A. J. Catal. 1981, 74, 129−135. (38) Chen, Y. X.; Chen, S. P.; Zhou, Z. Y.; Tian, N.; Jiang, Y. X.; Sun, S. G.; Dong, Y.; Wang, Z. L. J. Am. Chem. Soc. 2009, 131, 10860. (39) Jiang, P.; Zappone, M. W.; Bernasek, S. L.; A. Robertson, J. J. Vac. Sci. Technol., A 1996, 14, 2372−2377. (40) Feibelman, P. J.; Hammer, B.; Norskov, J. K.; Wagner, F.; Scheffler, M.; Stumpf, R.; Watwe, R.; Dumesic, J. J. Phys. Chem. B 2001, 105, 4018−4025. (41) Stroppa, A.; Termentzidis, K.; Paier, J.; Kresse, G.; Hafner, J. Phys. Rev. B 2007, 76, 195440.

CZ.1.07/2.3.00/20.0017 of the Ministry of Education, Youth and Sports of the Czech Republic), the Academy of Sciences of the Czech Republic (project KAN115600801), and the Czech Science Foundation (project P208/12/G016).

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