J. Phys. Chem. C 2010, 114, 21405–21410
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Density Functional Theory Study of Water Dissociative Chemisorption on the Fe3O4(111) Surface Chenggang Zhou,†,‡ Qingfan Zhang,† Lei Chen,† Bo Han,† Gang Ni,† Jinping Wu,† Diwakar Garg,§ and Hansong Cheng*,†,‡ Sustainable Energy Laboratory, China UniVersity of Geosciences, Wuhan 430074, Hubei, People’s Republic of China, Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, and Air Products and Chemicals, Inc., 7201 Hamilton BlVd., Allentown, PennsylVania 18195-1501, United States ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: October 25, 2010
Water dissociative chemisorption on the Feoct-tet1-terminated Fe3O4(111) surface was studied using periodic density functional theory (DFT) at both low and high water coverage. The active sites and adsorption patterns were identified, and the dissociation pathways and energetics were calculated. It was found that water can undergo dissociative chemisorption to form a surface hydroxyl group and a H atom with a favorable thermochemical energy and a moderate activation barrier at low coverage. This reaction can be readily catalyzed by water molecules around the active sites. We found that direct breakup of the hydroxyl group into H and O adatoms on the surface is energetically difficult and higher water coverage has an only modest effect on catalyzing the reaction. Our results are consistent with the kinetic and isotope exchange experiments. I. Introduction As a part of industrial steam methane reforming (SMR) processes, the water gas shift (WGS) reaction, in which CO reacts with water steam to produce hydrogen and CO2, has become a subject of increasing interest.1-8 The design and development of more efficient catalysts to enhance the production process have become essential to meet the demand for hydrogen, which has risen rapidly in recent years. The majority of the current research efforts on the WGS reaction is to use supported precious metals as catalysts to produce a high purity of hydrogen at temperatures lower than the temperature range used in industrial SMR processes.6,9 In contrast, a commercial scale WGS reaction utilizes mostly iron-based catalysts.1,10 In general, a WGS process consists of two stages. At the high temperature stage (320-450 °C), the shift occurs at a high conversion rate on the iron-based catalyst. At the low temperature stage (200-250 °C), the copper-based catalysts convert the majority of the remaining CO into CO2.1 Water dissociation has been identified to be one of the rate-limiting steps for the process.11-15 The purpose of the present study is to unravel the mechanism of water dissociative chemisorption on the Fe3O4(111) surface, which has been found to be the active phase during the WGS reaction at high temperature.16 Understanding water dissociation behavior is essential for design and development of novel catalysts to achieve high H2 production efficiency. Fe3O4 has a cubic inverse spinal structure where the tetrahedral sites are formally occupied by Fe3+ ions and the octahedral sites are occupied equally by Fe2+ and Fe3+ ions. It has been shown that the predominant natural growth facet of Fe3O4 is the (111) orientation.17-19 As shown in Figure 1, six facets of nonequivalent ideal bulk terminations can be created by cleaving the bulk along the (111) orientation. These include exposure of the following: * Corresponding author. E-mail:
[email protected]. † China University of Geosciences. ‡ National University of Singapore. § Air Products and Chemicals, Inc.
Figure 1. Side view of the Fe3O4(111) (2 × 2 × 2) unit cell.
1. Tetrahedral coordinated Fetet1 atoms; 2. Tetrahedral coordinated Fetet2 atoms; 3. Octahedral coordinated Feoct1 atoms; 4. Octahedral coordinated Feoct2 atoms; 5. Closed packed O1 layers; 6. Closed packed O2 layers. Somorjai and coworkers20 have shown that the energetically most favorable facets are Feoct2-tet1-terminated and Fetet1terminated surfaces that cut across the iron multiplayer due to the fact that such terminations result in breaking only five bonds compared to seven or nine bonds for the cleavage between one oxygen layer and one iron layer, which gives rise to a denser monolayer or multilayers. Experimentally, studies based on low-energy electron diffraction (LEED)21,22 suggested that surfaces prepared layer-bylayer epitaxially are Fetet1-terminated. However, a scanning tunneling microscopic (STM) study17 using a natural Fe3O4 single crystal reported that the Feoct2-tet1-terminated surface is more favorable. Theoretically, the ab initio Hartree-Fock calculations by Somorjai et al.20 suggested that a Feoct2-tet1terminated surface is energetically favorable. Zhu and coworkers performed density functional theory (DFT) calculations based on a generalized gradient approximation (GGA) and local density approximation (LDA) + U on five possible terminations of the low index polar (111) surface of Fe3O4 and observed strong surface relaxations upon geometry optimization.23 On the basis of the calculated surface free energies, they concluded
10.1021/jp105040v 2010 American Chemical Society Published on Web 11/17/2010
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that the Feoct2-tet1-terminated surface is indeed most stable. DFT calculations of surface energies by Grillo et al. indicated that at 298 and 1200 K the most stable equilibrium structure of Fe3O4(111) surface is Fetet1-terminated and the Feoct2-tet1terminated surface becomes more stable under oxygen-deficient conditions at high temperatures.24 The difference of the calculated surface energies of the Fetet1-terminated and the Feoct2-tet1terminated surfaces was found to be relatively small. Interactions between water and iron oxide have been investigated in several studies in the past few years.24-28 An ultraviolet photoelectron spectroscopic study on water adsorption on the Fe3O4(111) surface found three species on the surface: hydrogenbonded condensed ice, physisorbed monomeric water, and dissociatively chemisorbed water.25 It was proposed that water molecules first undergo a physisorption process and upon interaction between Fe 3d-orbitals and water O 2p-orbitals a weak substrate-adsorbate bond is created. Subsequently, water molecules dissociate into OH and H surface species, where the OH groups reside on the metal atoms and the H species are attached to the surface O atoms.26,27 Cutting and coworkers28 investigated water interaction with the Fe3O4(111) surface as a function of water partial pressure and temperature. They found that the dissociative chemisorption of water at 200 K is dependent on water coverage: below the partial pressure of 10-6 mbar, no dissociation was observed, and beyond that dissociation species can be detected. This suggests that interactions between water molecules play a critical role in promoting water dissociative chemisorption on Fe3O4 surfaces. A recent DFT study24 on water adsorption on the Fetet1-terminated surface reported that the initial water adsorption is dissociative and the surface species subsequently form a hydronium-ion-like bridge structure H3O+-OH-. However, the dissociation barrier was not reported, and thus it remains unclear whether the structure formation is kinetically facile. To date, water dissociative chemisorption on the Feoct2-tet1-terminated Fe3O4(111) surface has not been studied to our knowledge. Since this surface facet is energetically stable and possesses more chemically active sites, an understanding of water dissociative chemisorption on the surface is of fundamental importance to gain insight into WGS reaction process. In the present study, we investigate water-dissociative chemisorption on the Feoct2-tet1-terminated Fe3O4(111) surface using periodic density functional theory.29 The surface was described using a slab model with Fe3O4 stoichiometry. We first examined the preferred adsorption sites for water predissociation. We then investigated the minimum energy pathways that lead to water dissociative chemisorption. Both thermochemical energies and activation barriers were evaluated. In particular, attention has been paid to address the role of water in catalyzing the dissociative chemisorption. We show that the dissociation of a single water molecule requires considerable activation energy. However, the barrier of the reaction diminishes almost completely upon interacting with other water molecules. Further dissociation of the surface hydroxyl groups on the Feoct2-tet1terminated Fe3O4(111) surface was found to be energetically difficult. Our results are consistent with the available experimental facts to date. II. Computational Model and Method The present study employed a slab model with Feoct2-tet1termination to represent the Fe3O4(111) surface, which contains stoichiometrically eight layers of iron atoms and eight layers of oxygen atoms with a (1 × 1) unite cell. The unusually thick slab model is necessary to correctly model surface reactions
Zhou et al. without giving rise to artificial surface relaxation, as has been shown previously for water dissociative chemisorption.24 The top eight layers of the surface were fully relaxed upon the geometry optimization, and the bottom eight layers were kept fixed. The vacuum between adjacent slabs was set to be 13 Å to minimize the interaction between slabs. All electronic structure calculations were carried out using density functional theory method under the GGA with the exchange-correlation functional proposed by Perdew and Wang (PW91) as implemented in the VASP code.30,31 For a correlated system like Fe3O4 one would expect GGA + U to be more appropriate to describe electronic energies. However, upon structure optimization of H2O adsorption at the Feoct2 site using GGA + U, we found that it is nonbonding, while the adsorption structure remains similar to what was obtained with GGA (see Supporting Information). Since water adsorption on Fe3O4 was an observed phenomenon,24-27,32,33 we conclude that GGA is more suitable for describing H2O adsorption on the chosen Fe3O4 surface. The interactions between core electrons and ions were described by the projector augmented wave (PAW) method, and the valence electronic states were represented with a plan-wave basis set with the energy cutoff of 400 eV. Electronic energies were calculated with the SCF tolerance of 5 × 10-5 eV. A spinpolarization scheme was utilized to deal with the electronically open shell system intrinsic to Fe3O4. The Brillouin zone integration was performed using a grid of 3 × 3 × 1 MonkhorstPack special k-points. Structure optimization was carried out using the conjugate gradient algorithm. The tolerance set up for geometry optimizations is that the force near the equilibrium is less than 5 × 10-4 eV/Å. To test the computational accuracy, we calculated the lattice constant of bulk Fe3O4, yielding a value of 8.14 Å, in reasonable agreement with the experimental value of 8.39 Å.34 The calculated cohesive energy is 36.0 eV per Fe3O4 unit, compared well with the experimental value of 34.618 eV.20,35 The present study requires extensive search for transition states (TS) for a given reaction pathway. This was carried out by employing the “climbing images” nudged elastic band (CINEB) algorithm.36,37 Typically, eight images were produced between the states of reactant and product in each elementary process as the initial guesses for the reaction coordinates. For the TS search, the same force tolerance as the structural relaxation was used. Subsequently, each individual image was optimized based on the NEB algorithm. The adsorption energy of predissociation water is defined by:
∆Eads(nH2O) ) E(H2O/surf) - E(surf) - nE(H2O)
(1) where E(H2O/surf) and E(surf) are the total energies of the chosen surface with and without water, respectively, and E(H2O) is the energy of a water molecule. Here n represents the number of the adsorbed H2O molecules. To describe water dissociative chemisorption to form coadsorption H and OH species, we similarly define the chemisorption energy as:
∆Ediss(H2O) ) E(H/surf + OH/surf) - E(H2O) E(surf) (2) where E(H/surf + OH/surf) is the total energy of the coadsorbed H and OH species on the chosen surface.
Water Dissociative Chemisorption on Fe3O4(111)
Figure 2. Optimized structure of Feoct2-tet1-terminated surface. Side view (a) and top view (b).
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Figure 3. Electron density difference contours cut in parallel to the (111) plane across the center of the Feoct2 (a) and Fetet1 (b) atoms. Blue color represents the loss of electrons, and red color indicates the gain of electrons.
TABLE 1: Calculated Bader Charge of Fe Atoms in Bulk and Surface site
bulk (e)
surface (e)
Feoct2 Fetet1 Feoct1 Fetet2
1.27 1.47 1.43 1.47
1.06 1.08 1.31 1.36
III. Results and Discussion We first optimize the structure of the Feoct2-tet1-terminated surface, which is displayed in Figure 2. Compared with the structure with bulk bond parameters shown in Figure 1, considerable surface relaxation is observed with gradual shrinkage of the top layers. The Feoct2 and Fetet1 layers move inward to the underneath oxygen layer by as much as 18% and 70%, respectively, relative to their bulk positions. The vertical distance between the Fetet1 layer and the underneath oxygen layer is only 0.168 Å, indicating that the Fetet1 atoms are largely embedded in the oxygen layer. The strong inward surface relaxation has also been observed previously in several experiments and theoretical calculations.20,22,23 Ahdjoudj and coworkers performed full Fe6O8 slab relaxations using the Hartree-Fock theory and reported that the Feoct2 and Fetet1 layers move inward by as much as 36% and 57%, respectively. DFT calculations by Zhu and coworkers on the (111) surface also indicate strong inward relaxations of the Feoct2 layer by 62% and the Fetet1 layer by 36%. Experimentally, substantial relaxation of the Fetet1terminated surface was reported by Ritter and Weiss based on their LEED intensity analysis.22 The outmost Fetet1 layer was found to move inward by 41 ( 7%. Formally, the tetrahedral sites are occupied by Fe(III) atoms, and the octahedral sites are occupied by equal numbers of Fe(II) and Fe(III) atoms. We carried out population analysis based on the Bader charge division scheme on both the bulk and the surface. The calculated results are listed in Table 1. In the bulk, both Fetet1 and Fetet2 atoms are at high oxidization states, and Feoct2 atoms are at low oxidization states, qualitatively consistent with the formal charge assignment. Upon cleavage along the (111) orientation, all iron atoms are somewhat reduced relative to their bulk states due to the structural relaxation. In particular, the outmost of the surface atoms, Feoct2 and Fetet1, has been heavily reduced as their coordination numbers decrease from 6 to 3 and from 4 to 3, respectively. Figure 3 depicts the calculated electron density difference of the Feoct2-tet1-terminated surface, which is defined as the difference between the calculated total electron density of the surface
Figure 4. Optimized structures of a water molecule adsorbed at (a) the Feoct2 site and (b) the Fetet1 site.
and the electron density of the constituent atoms in their superpositions. The contours are cut in parallel to the (111) plane across the center of the Feoct2 (Figure 3a) and Fetet1 (Figure 3b) atoms, respectively. Clearly, the topmost of the surface is positively charged as illustrated in Figure 3a, making the Feoct2 atoms a primary target for water adsorption. The second topmost layer consists of both Fe and O atoms. Here, charge separation is clearly visible with the O atoms surrounded by abundant electrons and the Fetet1 atoms being highly charge deficient. The active reaction sites for water molecules are obvious: the O atom of water adsorbs on the cationic sites while the H atoms interact with the surface O atoms via H-bonding. We now explore possible configurations of predissociative adsorption of water on the Feoct2-tet1-terminated Fe3O4(111) surface. In general, water prefers to interact with the surface atoms with a high electron deficiency. As shown in Figure 4, there are two adsorption sites for water: the Feoct2 atom and the Fetet1 atom. On the Feoct2 atom, the water molecule is anchored on the surface, forming two strong bonds: the O atom of the water forms a covalent bond with the Fe atom with a distance of 2.071 Å, and one H atom forms a H-bond with a surface O atom with a short distance of 1.579 Å. The strong H-bonding gives rise to a significant OH bond stretch with the O-H distance elongating from 0.973 to 1.055 Å, indicating that the bond is somewhat activated. This interesting adsorption configuration allows the Fe atom to be further oxidized relative to its surface state, transferring 0.09 e to the O atom of water, which enables the O atom to withdraw less charge from H atoms. Consequently, the looser O-H bond allows the H atom to interact with the surface O atom more strongly, returning the extra charge gained from the O atom of water to the O atom of the surface. Overall, the charge of water remains neutral. Figure 5 displays the calculated density of states spectra. The low lying states of the surface are dominated by d orbitals of Fe. Upon water adsorption, the d band is pushed up slightly
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Figure 5. Calculated density of states of (a) bare surface, (b) water adsorption at the Feoct2 site, and (c) water adsorption at the Fetet1 site.
Figure 6. Optimized structures of two water molecules coadsorbed on the surface. (a) The most stable structure with the second water molecule forming H-bonding with the first one and surface O atom. (b) Two water molecules occupy the two surface Fe sites.
above the Fermi level due to the charge transfer from the Feoct2 atom to the O atom of water, while the p band of O atoms moves downward upon accepting electron. The calculated adsorption energy of -1.11 eV at the Feoct2 site indicates a strong binding. For the adsorption at the Fetet1 site, the embedded Fetet1 atom is raised up considerably upon interacting with water as shown in Figure 4b. The water molecule tilts from the surface with the O atom attached to the Fetet1 atom with a distance of 2.320 Å, which is significantly longer than the value for the adsorption at the Feoct2 site, suggesting a weaker bonding. One H atom points to the surface O atom with a distance of 1.476 Å, indicating a strong H-bonding interaction. Similar to the case of adsorption at the Feoct2 site, electron transfer occurs simultaneously from the Fetet1 atom to the water O atom and from the H atom of water to the surface O atom via H-bonding. As a consequence, the d band of Fe shifts slightly to the virtual orbitals, and the p orbitals of O atoms move to a lower energy level (Figure 5). The calculated adsorption energy at the Fetet1 site is -0.67 eV, much lower than the value at the Feoct2 site. The reason is primarily that, at the Feoct2 site, the Fe atom is more exposed to water, while at the Fetet1 site, the metal atom is surrounded by surface O atoms, which prevents water from close contact with the Fe atom. We next explore the adsorption pattern of higher water coverage by placing another water molecule on the Feoct2-tet1terminated surface. Of numerous adsorption configurations sampled, two stable structures, shown in Figure 6, were obtained. One is of the lowest potential energy with the Feoct2 site occupied by a water molecule with the Fe-O distance of 2.003 Å (Figure 6a). Interestingly, the second water molecule prefers to stay away from the Fetet1 site. It forms strong H-bonds with
Zhou et al.
Figure 7. Calculated energy profile of water dissociative chemisorption with the optimized structures of the initial, transition, and final states. The asterisks here denote the adsorption species. The potential energy of water on the bare surface is set to zero.
both the first water and the surface O atom with the optimized H-bond distances of 1.462 and 1.509 Å, respectively. The second coadsorption structure, shown in Figure 6b, is that the two water molecules separately occupy the surface Fe sites with a weaker H-bond between the water molecules, evidenced by the longer O-H distance of 1.873 Å. Again the Fetet1 atom is pulled out of the surface considerably upon water adsorption. Both Fe-O distances are significantly longer than the Feoct2-O distance shown in Figure 6a. This structure is energetically less stable than the first one by 0.51 eV. Therefore, we will focus our discussions on water dissociative chemisorption only on the first structure. We next investigated monomeric water dissociative chemisorption based on the most stable structure shown in Figure 4a. The strong adsorption of the predissociation state of water allows the O-H bond to be activated, leading to the migration of the H atom to the surface O atom to form an H adatom and a hydroxyl species. The main optimized structural parameters of the TS and the final product and the calculated energy diagram are shown in Figure 7. Relative to the predissociation state, the breakup of water is a slightly exothermic process with the calculated thermochemical energy of only -0.22 eV. The calculated activation barrier is 0.71 eV, suggesting that the dissociation might be difficult at low temperatures. Previous experiments suggested that there is an activation barrier associated with water dissociation on the surface, although the exact value of the barrier was not reported.24,25 Furthermore, we note that the dissociative chemisorption energy of water on the Feoct2-tet1-terminated surface is -1.33 eV, considerably larger than the calculated value of -0.99 eV for the same process on the Fetet1-terminated surface reported by Grillo and coworkers.24 This implies that the Feoct2-tet1-terminated surface may be chemically more active than the Fetet1-terminated surface. A previous theoretical study suggested that the H adatom is mobile on the Fe3O4(111) surface via the H-bonding network.24 This could make the surface O site available to assist further dissociation of the hydroxyl group. The internal rotation of the Fe-O bond allows the hydroxyl group to readily reorient itself from tilting up to pointing down to the surface O atom (
