Density Functional Theory Study of NHx (x = 0−3) and N2 Adsorption

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Density Functional Theory Study of NHx (x ) 0-3) and N2 Adsorption on IrO2(110) Surfaces Chia-Ching Wang, Shih Syong Siao, and Jyh-Chiang Jiang* Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, 43, Keelung Road, Section 4, Taipei 106, Taiwan ReceiVed: July 21, 2010; ReVised Manuscript ReceiVed: September 21, 2010

The oxidation of ammonia (NH3) and the reduction of nitrogen (N2) are two important processes in chemistry. In this study, we used density functional theory calculations to investigate the adsorptions of NHx (x ) 0-3) and N2 on IrO2(110) surfaces, with density of states (DOS) analysis providing information relating to bond character and state interactions. These adsorbates have higher binding energies on the IrO2(110) surface than on the RuO2(110) surface because the former forms stronger σ bonds with the adsorbed molecules. The surface adsorptions of NH2 and NH on the IrO2(110) surface proceed with similar binding energies and similar hybridizations of the nitrogen atoms. In addition, the orientations of NH2 and NH adsorbed on the IrO2(110) surface are governed by lateral interactions with surface oxygen atoms (Ocus or Obr), rather than by hydrogen bonding. We calculated the binding energy for the adsorption of N2 on the IrO2(110) surface to be 1.10 eV. The weakening of the NtN triple bond was evident from our DOS results; strong bonding forces, including σ- and π-type interactions, exist between the N2 molecule and the surface, suggesting that N2 molecules are moderately activated by IrO2(110) surfaces. 1. Introduction Iridium dioxide (IrO2), which crystallizes in the tetragonal rutile structure,1 is a metal oxide that exhibits metallic conductivity at room temperature.2 Its interesting electrochemical properties have led to IrO2 being used in several applications, including electrode materials for chlorine or oxygen evolution,3-5 optical switching layers in electrochromic displays,6 and pHbased electrodes.7,8 The properties of IrO2 share many similarities with those of ruthenium dioxide (RuO2). The Rucus atoms (cus ) coordinatively unsaturated site) on the RuO2(110) surface can behave as highly active sites for catalytic reactions, such as the oxidations of CO9 and HCl.10-12 The (110)-oriented domain is one of the dominative surfaces of IrO2;13-16 this IrO2(110) surface has been characterized in recent studies, both by experimentation and with use of density functional theory (DFT).13,17 Figure 1 presents an atomic model of the stoichiometric IrO2(110) surface [s-IrO2(110)], which presents exposed rows of twofold-coordinated bridge oxygen atoms (Obr) and fivefold-coordinated Ir atoms along the [001] direction. Additional oxygen atoms, Ocus, may be adsorbed on the Ircus atoms; the surface is considered to be an oxygen-rich IrO2(110) surface [o-IrO2(110)] when Ocus atoms exist on the surface. The decomposition of ammonia (NH3) is an important industrial reaction. The catalytic oxidation of NH3 to NO, the so-called Ostwald process, is a key step in the production of nitric acid. In addition, the oxidation of NH3, leading to N2 and H2O, has become increasingly attractive in connection with the removal of NH3 from waste streams.18 Many reports describe the decomposition and oxidation of NH3.19-25 In addition, the reduction of N2 to yield NH3 (so-called “nitrogen fixation”) is one of most important topics in chemistry and biology.26-28 In biological systems, N2 fixation is mediated under ambient conditions by the enzyme nitrogenase.29-32 In contrast, industrial * To whom correspondence should be addressed. Phone: +886-227376653Fax: +886-2-27376644. E-mail: [email protected].

Figure 1. Ball-and-stick model of the stoichiometric IrO2(110) surface.

N2 hydrogenation (i.e., the Haber-Bosch process) is performed under drastic conditions of high temperatures and high pressures. In recent years, many computational and experimental studies have been performed to develop low-molecular-weight transition metal complexes that mimic the structural or functional role of nitrogenase.26,33-39 The first step in all N2 fixation reactions is activation of bonded N2; because N2 is very inert, the search for a catalyst that could activate it remains a considerable challenge. The most critical factor in determining the reactivity and selectivity of the reactions of NH3 and N2 is the nature of the catalyst. Recent experimental25 and DFT40-42 studies have found that RuO2(110) surfaces have excellent ability to mediate the oxidation of NH3, with the selectivity toward the oxidation products being controlled by the coverage of surface Ocus atoms. The DFT-calculated results for the RuO2(110) surface motivated us to explore the catalytic properties of the IrO2(110) surface because these two surfaces share many similarities. In this study, we calculated the adsorptions of NHx (x ) 0-3) species and

10.1021/jp1067846  2010 American Chemical Society Published on Web 10/12/2010

NHx (x)0-3) and N2 Adsorption on IrO2(110) Surfaces

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N2 molecules onto IrO2(110) surfaces. We analyzed the density of states (DOS) of each adsorbed species and obtained detailed information regarding the interactions between the adsorbates and the surface. The interactions between NHx species and IrO2(110) surfaces are quite complicated: both Ir-N bonding and lateral interactions play important roles. In the adsorption of N2, strong interactions with the IrO2(110) surface result in weakening of the NtN triple bond, thereby moderately activating the N2 molecule. 2. Computational Details DFT calculations in this study were performed using the Vienna Ab Initio Simulation Package (VASP).43,44 The generalized gradient approximation (GGA) was used with the function described by Perdew and Wang45 and a cutoff energy of 300 eV. Electron-ion interactions were investigated using the projector augmented wave method.46 The IrO2(110) surfaces were modeled as a two-dimensional slab in a three-dimensional periodic cell; a 14 Å vacuum space was introduced to curtail interactions between the slabs. Each slab was a 2 × 1 surface having a thickness of four layers, where one layer is defined as one O-Ir-O repeat unit; the lowest layer was fixed during the structure optimization. The [001], [1j10], and [110] directions of the slab were defined as the x, y, and z dimensions of the supercell. The k-points of 3 × 3 × 1 were set by MonkhorstPack. More details of the surface model and the benchmark of different cutoff energy and k-point settings are available in the Supporting Information. All the optimized structures were checked for their validity using normal-mode frequency analysis. For a real minimum structure, all frequencies must be positive. In the vibrational frequency calculations, only the upper two atomic layers of the surface and the adsorbates were relaxed. Recently, DFT calculations of IrO2(110) surfaces using the same parameters were applied to identify the core-level spectral features during surface deoxygenation processes.17 The binding energies (Eb) of NHx and N2 on the surfaces were calculated using the formula

Eb ) (ES + EM) - EM/S where ES is the energy of the clean IrO2(110) surface (stoichiometric or oxygen-rich surface), EM is the energy of a single adsorbate molecule, and EM/S is the total energy of the NHx or N2 adsorption model. A positive value of Eb indicates an exothermic chemisorption process. 3. Results and Discussion 3.1. NHx Adsorption. Figure 2 presents top views of NHx (x ) 0-3) species adsorbed on an s-IrO2(110) surface with the coverage (θ) of 0.5 ML (a-d) and on an o-IrO2(110) surface (e-j). Table 1 lists the binding energies and selected geometric parameters of these adsorption structures. In the naming of the adsorption structures, the label “s” means that the NHx species is adsorbed on the stoichiometric surface and “o” denotes adsorption on an oxygen-rich surface. For the adsorption of NH2 and NH on IrO2(110) surfaces, the notation “-cus” implies that the N-H bond of the molecule points in the [001] direction, i.e., the direction of a neighboring Ircus or Ocus atom; in contrast, “-br” means that the N-H bond points in the [1j 10] direction, i.e., toward the Obr atom. Of the NHx (x ) 0-3) species on the s-IrO2(110) surface, NH3 has the lowest binding energy (2.14 eV) and a single N atom has the highest (3.69 eV). In the adsorption of NHx species, as each hydrogen atom is removed

Figure 2. Top views of NHx species adsorbed on the stoichiometric IrO2(110) surface [(a) s-NH3, (b) s-NH2-cus, (c) s-NH-cus, (d) s-N] and on the oxygen-rich IrO2(110) surface [(e) o-NH3, (f) o-NH2-cus, (g) o-NH2-br, (h) o-NH-cus, (i) o-NH-br, (j) o-N].

TABLE 1: Binding Energies (eV) and Selected Structural Parameters (Å) of NHx (x ) 0-3) and N2 Molecules Adsorbed on IrO2(110) Surfaces model s-NH3 s-NH2-cus s-NH-cus s-N o-NH3 o-NH2-cus o-NH2-br o-NH-cus o-NH-br o-N

Eb 2.14 3.13 3.29 3.69 2.65 3.36 3.26 3.24 3.08 3.87

d(Ir-N) 2.09 1.92 1.84 1.71 2.10 1.92 1.92 1.83 1.83 1.70

d(O · · · H)a

d(N-H) b

2.43

1.03, 1.04b 1.03 1.03 1.06 1.06

2.10 2.35 2.66 2.01 2.44

1.03, 1.04 1.02 1.05

a Hydrogen bond distance between the H atom on the NHx species and a neighboring Ocus (Obr) atom. b Length of the N-H bond undergoing hydrogen bonding with a neighboring Ocus (Obr) atom.

from NHx, the Ir-N bond order increases and the corresponding Ir-N bond length decreases. On the o-IrO2(110) surface, the binding energies of the NHx species vary insignificantly relative to those on the s-IrO2(110) surface, except for NH3, which can form a hydrogen bond with the Obr or Ocus atom; notably, hydrogen bonding between NH3 and the Obr atom has been characterized previously from an electron density difference contour plot.41 For model s-NH3 and o-NH3, the N-H bonds undergoing hydrogen bonding are longer than the other N-H bonds. Although both the Obr and Ocus atoms behave as proton acceptors during hydrogen bond formation, the shorter distance between the Ocus atom and the H atom means that NH3 on the o-IrO2(110) surface forms a stronger hydrogen bond with the Ocus atom; this interaction causes the binding energy of NH3 to increase by 0.51 eV relative to that on the s-IrO2(110) surface.

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Figure 4. Molecular DOS of the NHx (x ) 1-3) species and atomic DOS of the N atom before (gray lines) and after (black lines) adsorption on the IrO2(110) surface. (a) s-NH3 (C3V), (b) s-NH2-cus (C2V), (c) s-NHcus (Cs), and (d) s-N. Figure 3. (a) Projected DOS of the dz2 states in Ircus (black line), Rucus (gray line), and Ticus (dash line) atoms. (b) Schematic representation of gas-surface interactions between the 2a1 orbital of the NH3 molecule and the dz2 states of Mcus atoms (M ) Ir, Ru, Ti).

The interactions between NH3 and IrO2(110) surfaces are quite strong; indeed, the binding energy of 2.14 eV is much higher than the adsorption on the Ir(100) surface47 and on other transition metal,48-51 metal oxide,41,52,53 and nonmetal surfaces.54,55 The comparison of binding energy of NH3 adsorption on different surfaces is made in the Supporting Information. In comparison with the results of our previous study,41 the adsorptions of NHx species on IrO2(110) surfaces exhibit bonding characteristics similar to those on RuO2(110) surfaces. Indeed, the binding energy differences between NHx species on both IrO2(110) and RuO2(110) surfaces are similar. For example, the differences between the binding energies of NH3 and NH2 on the s-IrO2(110) surface (θ ) 0.5 ML) and on the RuO2(110) surface are 0.99 and 0.91 eV, respectively; for NH2 and NH, these values are 0.16 and 0.18 eV, respectively. The major difference between the NHx adsorptions on these two oxide surfaces is that all of the binding energies on the IrO2(110) surfaces are higher than those on the RuO2(110) surfaces, presumably because of differences in the strength of σ bonding. The adsorptions of NH3 on the IrO2(110) and RuO2(110) surfaces both occur through single dative bonds, but this association is 0.58 eV more stable on the s-IrO2(110) surface (θ ) 0.5 ML). Taken together, our results suggest that IrO2(110) surfaces form stronger σ bonds with NHx species. In a previous study,40 we found that σ-bond formation between NHx species and RuO2(110) surfaces occurs through one of the molecular orbitals in NHx interacting with the dz2 state of surface Rucus atoms. On the IrO2(110) surface, the adsorptions of NHx species occur through the same types of state interactions for σ-bond formation as those on RuO2(110) surfaces. Figure 3a displays the projected density of states (PDOS) of the dz2 states of the Ircus and Rucus atoms, as well as that of the Ticus atom in the rutile-crystallized s-TiO2(110) surface for comparison.56 The distribution of the vacant dz2 state of the Ircus atom is located at

the lowest energy region, and is closer to the Fermi energy. This lower vacant dz2 state will be closer to the highest occupied molecular orbitals (HOMOs) of the NHx species prior to adsorption, thereby inducing stronger state interactions as the σ bonds form. Figure 3b presents the gas-surface state interactions between the HOMO, the 2a1 orbital (the lone pair; the symmetry is defined in the next paragraph), of NH3 and the dz2 states of Mcus (M ) Ir, Ru, Ti) atoms. The dz2 state of the Ircus atom displays the largest magnitude of splitting after interacting with NH3; we observed the highest stabilizing energy after NH3 adsorbed on the Ircus atom. On the other hand, the higher state energy of dz2 in the Rucus atom results in a smaller dz2 state splitting, less stabilizing energy, and lower binding energies for NH3 molecules on RuO2(110) surfaces.41 Correspondingly, the highest energy of the dz2 state in the Ticus atom also results in a low binding energy (0.90 eV) for NH3 molecules on TiO2(110) surfaces. Figure 4 illustrates the molecular DOS of free NHx (gray line) and adsorbed NHx (black line) molecules on an s-IrO2(110) surface. In this study, we assign the specific symmetries for the NH3, NH2, and NH adsorption systems as C3V, C2V, and Cs, respectively. By comparing the binding energies of NH2 and NH and their corresponding DOS plots on the s-IrO2(110) surface, we find more state interactions in the Figure 4c (NH) than in Figure 4b (NH2); nevertheless, the binding energy of the NH molecule is just slightly higher than that of the NH2 molecule. Upon proceeding from the model s-NH2-cus to the model s-NH-cus, one H atom is removed and the value of d(Ir-N) decreases from 1.92 to 1.84 Å, implying an increase in the bond order and bond energy. The binding energy of the NH molecule is, however, only 0.16 eV higher than that of the NH2 molecule. After adsorption, the 2a1 orbital of the NH2 molecule and the 3a′ orbital of the NH molecule will interact with the dz2 state of the Ircus atom to form σ bonds. In addition to σ bonding, the b2 orbital of the NH2 molecule will interact with the dyz state of the Ircus atom to form a π bond; Figure 4b clearly reveals that the unique b2 orbital of the NH2 molecule has disappeared. For the adsorption of the NH molecule, the 2a′ and a′′ orbitals will also interact with the dxz and dyz states

NHx (x)0-3) and N2 Adsorption on IrO2(110) Surfaces

Figure 5. π-Type state interactions between the adsorbates and Ircus atoms in the adsorption models (a, b) s-NH2-cus and (c, d) s-NHcus.

of the surface Ircus atom (Figure 4c). The change in the Ir-N bond length and the DOS analyses confirm that the NH molecule has a higher Ir-N bond order than does the NH2 molecule. In the model s-NH-cus, the angle ∠(Ir-N-H) is 111.3°; the nonvertical N-H bond in the model indicates that the hybridization of the adsorbed NH molecule is similar to that of the adsorbed NH2 molecule. That is, the electronic structure of the adsorbed NH molecule is closer to sp2 hybridization. The PDOS plot in Figure 5 reveals the π-type state interactions of the models s-NH2-cus and s-NH-cus. Both the NH2 and NH molecules exhibit very good state overlap with the dyz state of the Ircus atom for π-bond formation (Figure 5a,c). For the NH molecule, the molecular 2a′ orbital also interacts with the dxz state of the Ircus atom to form another π-type interaction in the x direction. The degree of overlap between the 2a′ state and the dxz state in Figure 5d is not, however, as good as those in Figure 5a,c (π bonding in the y direction). On the other hand, in Figure 5b, although the b1 orbital of the NH2 molecule retains its molecular character, it still undergoes a weak interaction with the dxz state of the Ircus atom. Thus, the hybridizations of the NH2 and NH molecules both lie somewhere between sp and sp2; the adsorptions of the NH2 and NH molecules occur with similar electronic interactions, but their different degrees of π-type interactions in the x direction result in different binding energies. Another reason for the similar binding energies of NH2 and NH is that the N-H bond of the NH molecule is weakened when it is adsorbed on IrO2(110) surfaces, making the NH molecule unstable. The strength of the N-H bond in the NH molecule is reduced when its 2a′ orbital interacts with the dxz state of the Ircus atom. In the model s-NH-cus, the value of d(N-H) of 1.05 Å is even greater than the bond length of the hydrogen-bonded N-H bond in the model o-NH3 (1.04 Å); such elongation of the N-H bond also indicates a weakened bond. The weakening of the N-H bond of the NH molecule is also found on the RuO2(110) surface. For NHx dehydrogenation reactions on RuO2(110) surfaces,40 the dehydrogenation of the NH molecule has a low reaction barrier relative to those of the NH2 and NH3 molecules. When all of the hydrogen atoms are removed, a pure triple bond can form between the nitrogen and

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Figure 6. π-Type state interactions in the x direction of the adsorption models (a) s-NH2-br, (b) s-NH-br, (c) o-NH2-br, and (d) o-NH-br.

iridium atoms, resulting in the highest binding energy on the IrO2(110) surface. For the adsorptions of NH2 and NH on the s-IrO2(110) surface, only the adsorption models s-NH2-cus and s-NH-cus (Figure 2b,c) are stable adsorption structures. On the stoichiometric surface, another possible adsorption direction of the NH2 and NH molecules, s-NH2-br and s-NH-br, are transition states (i.e., they are not energetically local minima). Vibration analysis revealed that these two adsorption structures are positioned at the first-order saddle point; the vibration modes of the imaginary frequencies of the models s-NH2-br and s-NH-br both represent hindered rotation along the Ir-N bonds. It is relatively unstable for the NH2 and NH molecules to be adsorbed on the s-IrO2(110) surface with their hydrogen atoms pointing toward neighboring Obr atoms, but if there are Ocus atoms on the surface (i.e., for the oxygen-rich surface), the adsorption models o-NH2-br and o-NH-br become stable structures. Therefore, the presence of Ocus atoms appears to be an important factor in stabilizing NH2 and NH molecules on o-IrO2(110) surfaces. Parts (a) and (b) in Figure 6 display the π-type state interactionssb2 (NH2) and a′′ (NH) orbitals interacting with the dxz state of Ircus atomsin the models s-NH2-br and s-NH-br. These two PDOS plots suggest that the π-bond interactions of s-NH2-br and s-NH-br in the x direction are not as strong as those in Figure 5a,c. In Figure 6c,d, however, the same π interactions with the dxz state in the models o-NH2-br and o-NH-br exhibit excellent state overlap. In addition, the atomic px orbital of the Ocus atom also overlaps with the states of the π-bond interaction in Figure 6c,d, implying that the Ocus atom not only induces better π-bond interactions but also is itself included in this π-bonding system. In fact, the kinds of interactions with the Obr atoms that stabilize the adsorbed NH2 and NH molecules are also present in Figure 5a,c, but the overlap regions are smaller. In the adsorption of NH3, hydrogen bonding with the Ocus or Obr atom plays an important role in increasing the binding energy. In the adsorptions of NH2 and NH molecules on o-IrO2(110) surfaces, the distances between the hydrogen atoms and the neighboring oxygen atoms [d(O · · · H)] are within a reasonable range for hydrogen bond formation. Notably, however, another lateral interaction, between the Ir-N π bonding state and the neighboring oxygen atoms,

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Figure 7. Geometry, bond lengths, binding energy, and N-N stretching frequency of the N2 molecule adsorbed on the IrO2(110) surface.

TABLE 2: Binding Energy (eV), N-N Stretching Frequency (cm-1), and Selected Geometric Parameters (Å) of N2 Adsorption on s-RuO2(110) and s-IrO2(110) Surfaces surface s-RuO2(110) s-IrO2(110)

c

Eb

d(M-N)a

d(N-N)

ν(N-N)

0.53 1.10

2.027 1.934

1.118 1.124

2280 2199

a M ) Rucus or Ircus. b Calculated ν(N-N) in the gas phase is 2398 cm-1. c Reference 40.

appears to be a more important factor affecting the adsorption direction of the NH2 and NH molecules. 3.2. N2 Adsorption. Figure 7 displays the adsorption geometry of the N2 molecule on the s-IrO2(110) surface. The adsorbed N2 molecule stands vertically on the Ircus atom in a monometallic end-on binding mode. We did not observe other binding modes, such as bimetallic end-on or side-on modes,26 for the IrO2(110) surface. The binding energy of the N2 molecule on the s-IrO2(110) surface is 1.10 eVsmuch higher than that on the s-RuO2(110) surface (0.53 eV)40 and higher than those calculated recently on different catalytic materials for the same binding mode.36,57,58 The binding energy, N-N stretching frequency, and selected geometric parameters of the N2 molecule adsorbed on s-IrO2(110) and s-RuO2(110) surface are summarized in Table 2. In this study, we also calculated the adsorption of N2 on the TiO2(110) surface for comparison, but found no stable adsorption structure. The N2 molecular binding energies on these three rutile-crystallized metal oxide (110) surfaces (IrO2, RuO2, and TiO2) follow the same trend as those for NH3. Again, the lower energy level of the dz2 vacant band in the Ircus atom will definitely cause a stronger σ-bond interaction with the adsorbates, as indicated in Figure 3. It is surprising that such a strong interaction exists between the N2 molecule and the IrO2(110) surface through simple monometallic end-on binding. Our calculations reveal that the N-N bond length (1.124 Å) of the adsorbed N2 molecule is 0.013 Å longer than that in the gas phase (1.111 Å) and that the N-N stretching frequency of 2199 cm-1 is red-shifted by 199 cm-1 from that in the gas phase (2398 cm-1). In addition, compared with the adsorptions of the NHx species on the s-IrO2(110) surface, the Ircus-N bond length of the adsorbed N2 (1.93 Å) lies between those of adsorbed NH3 (2.09 Å) and NH2 (1.92 Å), indicating that the interaction between the N2 molecule and the IrO2(110) surface involves not only σ bonding but also π-type interactions. According to the N2 activation scale classified by Studt and Tuczek,33 the degree of N2 activation induced by the IrO2(110) surface belongs in the category

Figure 8. (a) Molecular DOS of free (gray line) and adsorbed (black line) N2 on the IrO2(110) surface. (b) State interactions of the adsorbed N2 molecule (dashed line) with the dz2 (gray line) and dxz + dyz (black) states in the Ircus atom.

“moderately activated.” To obtain additional information regarding the bonding of the N2 molecule on the IrO2(110) surface, we plotted the DOS of the N2 molecule before and after its adsorption on the IrO2(110) surface (Figure 8a) and the overlap of the adsorbed N2 molecular orbitals and the Ircus atomic d states (Figure 8b). After the binding of N2 on the surface, the intensities of the degenerate π-bonding orbitals (1π and 2π) decreased significantly and extended into the distribution region. In Figure 8b, the 4σ* orbital of the N2 molecule overlaps strongly with the dz2 (gray line) state of the Ircus atom; in addition to this σ-bond interaction, π-bond interactions also exist between the N2 molecule and the Ircus atom. The combination of the dxz and dyz states of Ircus (black line) exhibits good overlap with the degenerate π states of N2. The existence of multistate interactions explains the high adsorption energy for the binding of N2 on the IrO2(110) surface. 4. Conclusion We have studied the adsorptions of NHx species and the N2 molecule on s- and o-IrO2(110) surfaces. DOS analyses provided detailed information characterizing the interactions between the adsorbates and these surfaces. Because the vacant dz2 state of the Ircus atom is distributed in the lower energy region, it interacts strongly with the HOMOs of the adsorbates, resulting in higher binding energies for NHx and N2 on IrO2(110) surfaces relative to those on RuO2(110) surfaces. On both s- and o-IrO2(110) surfaces, the adsorptions of NH3 have the lowest binding energies; when all the hydrogen atoms are removed, the single N atom has the highest binding energy. For the adsorptions of NH2 and NH, both molecules have hybridizations located somewhere between sp and sp2; a complicated set of interactions results in similar binding energies for these two molecules. Although the value of d(Ir-N) in the model s-NH-cus is smaller than that in the model s-NH2-cus, weakening of the N-H bond destabilizes the NH molecule and results in a binding energy similar to that of the NH2 molecule. In addition, the adsorptions

NHx (x)0-3) and N2 Adsorption on IrO2(110) Surfaces of NH2 and NH molecules reveal an interesting phenomenon: hydrogen bonding with surface oxygen atoms is not the only factor that stabilizes their adsorptions. According to DOS analysis, the specific p orbital of the Ocus or Obr atom could overlap with the Ir-N π bonding state during the adsorptions of NH2 and NH; these lateral interactions are even more important than hydrogen bonding. For the adsorption of the N2 molecule, DOS analysis suggests that the bonding of N2 to the Ircus atom occurs through σ and π interactions, thereby weakening the strength of the NtN triple bond. Our DFT calculations indicate that the N2 molecule is moderately activated by the IrO2(110) surface. Therefore, although it lacks the complicated structure of a metal complex, the IrO2(110) surface might find potential use as a catalyst for N2 fixation. Acknowledgment. We thank the National Science Council of Taiwan (NSC 98-2113-M-011-001-MY3 and NSC 94-2120M-011-001) for supporting this research financially and the National Center of High-Performance Computing and Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan, for computer time and facilities. Supporting Information Available: Table S1 lists the benchmark of the calculated binding energy of NH3-cus on IrO2(110) surface. Table S2 lists the comparison of calculated binding energy of NH3 on metal, metal-oxide, and nonmetal surfaces.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mattheiss, L. F. Phys. ReV. B 1976, 13, 2433. (2) Ryden, W. D.; Lawson, A. W.; Sartain, C. C. Phys. ReV. B 1970, 1, 1494. (3) Osaka, A.; Takatsuna, T.; Miura, Y. J. Non-Cryst. Solids 1994, 178, 313. (4) Ioroi, T.; Kitazawa, N.; Yasuda, K.; Yamamoto, Y.; Takenaka, H. J. Electrochem. Soc. 2000, 147, 2018. (5) Hansen, H. A.; Man, I. C.; Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Rossmeisl, J. Phys. Chem. Chem. Phys. 2010, 12, 283. (6) (a) Cogan, S. F.; Plante, T. D.; McFadden, R. S.; Rauh, R. D. Sol. Energy Mater. 1987, 16, 371. (b) Nishio, K.; Watanabe, T.; Tsuchiya, T. Thin Solid Films 1999, 350, 96. (7) Fog, A.; Buck, R. P. Sens. Actuators 1984, 5, 137. (8) Yao, S.; Wang, M.; Madou, M. J. Electrochem. Soc. 2001, 148, H29. (9) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (10) Lo´pez, N.; Go´mez-Segura, J.; Marı´n, R. P.; Pe´rez-Ramı´rez, J. J. Catal. 2008, 255, 39. (11) Crihan, D.; Knapp, M.; Zweidinger, S.; Lundgren, E.; Weststrate, C. J.; Andersen, J. N.; Seitsonen, A. P.; Over, H. Angew. Chem., Int. Ed. 2008, 47, 2131. (12) Zweidinger, S.; Crihan, D.; Knapp, M.; Hofmann, J. P.; Seitsonen, A. P.; Weststrate, C. J.; Lundgren, E.; Andersen, J. N.; Over, H. J. Phys. Chem. C 2008, 112, 9966. (13) He, Y. B.; Stierle, A.; Li, W. X.; Farkas, A.; Kasper, N.; Over, H. J. Phys. Chem. C 2008, 112, 11946. (14) Chen, R.-S.; Huang, Y.-S.; Tsai, D.-S.; Chattopadhyay, S.; Wu, C.-T.; Lan, Z.-H.; Chen, K.-H. Chem. Mater. 2004, 16, 2457. (15) Chen, R. S.; Chang, H. M.; Huang, Y. S.; Tsai, D. S.; Chattopadhyay, S.; Chen, K. H. J. Cryst. Growth 2004, 271, 105. (16) Jian, X.-H.; Tsai, D.-S.; Chung, W.-H.; Huang, Y.-S.; Liu, F.-J. J. Mater. Chem. 2009, 19, 1601. (17) Chung, W.-H.; Wang, C.-C.; Tsai, D.-S.; Jiang, J. C.; Cheng, Y.C.; Fan, L.-J.; Yang, Y.-W.; Huang, Y.-S. Surf. Sci. 2009, 604, 118.

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