Interaction of NO with RuO2 (110) Surface: A First Principles Study

Aug 2, 2007 - We report results of density functional theory calculations of the interaction of NO with the stoichiometric RuO2(110) surface that prov...
0 downloads 0 Views 440KB Size
Download PDF
J. Phys. Chem. C 2007, 111, 12361-12368

12361

Interaction of NO with RuO2(110) Surface: A First Principles Study Sampyo Hong and Talat S. Rahman* Department of Physics, UniVersity of Central Florida, Orlando, Florida 32816-2385

Karl Jacobi and Gerhard Ertl Department of Physical Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: March 14, 2007; In Final Form: June 12, 2007

We report results of density functional theory calculations of the interaction of NO with the stoichiometric RuO2(110) surface that provide insights into the experimentally observed lack of reactivity for the system. We find that NO adsorbs on top of the undercoordinated Ru (Ru-cus) with an upright axis, and the adsorption energy (with zero-point contribution) changes from 1.61 eV for 0.5 ML to 1.49 eV for 1 ML coverage. Once all Ru-cus sites are occupied, NO adsorbs on O-bridge sites with adsorption energy of 0.66 eV, forming an asymmetric O‚‚‚N-O surface complex. We also find a high dissociation barrier of 3.22 eV for NO on Rucus. Although the activation energy for oxidation of NO is calculated to be about 1.2 eV, the location of the final state makes the formation of NO2 only transient with a large probability of reverting back to NO. Additionally, the total energy difference for the reaction NO + NO f N2O + O on RuO2(110) is found to be about 1.35 eV. Comparison of results with those for a similar overlayer of CO on the surface show the NO-Ru-cus bond to be stronger than CO-Ru-cus, the difference arising from the contribution of the unpaired 2π* electrons for the former.

1. Introduction The examination of adsorption and reactions of NO on surfaces has drawn much attraction over the years not only because of its importance to ammonia oxidation chemistry, but also because NOx species coming from car engines contribute significantly to air pollution, being the major contributor to acid rain and photochemical smog. To understand and thereby find suitable and affordable catalysts that would help convert NO into more innocuous forms, experimental and theoretical work have focused mostly on metallic substrates1 and some on oxide surfaces.2-5 In this connection, recent findings of the enhanced catalytic activity of RuO2(110) toward CO oxidation6,7 have posed the question of whether it would behave similarly toward the oxidation of NO. For CO oxidation, metallic Ru is in fact practically inactive at low O2 pressure but under high O2 pressure displays excellent performance.8 This seemingly pressure gap has been attributed to the epitaxial growth of thin RuO2layers that have been found9-11 to form at high O2 pressure instead of a simple layer of adsorbed atomic oxygen on the surface. Bulk RuO2 has a rutile structure whose stoichiometric (110) surface is shown in Figure 1. Of particular relevance are the so-called coordinatively unsaturated sites (cus) denoted as Rucus, which are decisive for catalytic activity. In a recent study, it was found that NO chemisorbs strongly on RuO2(110); however, in contrast to CO no oxidation was observed.12 In terms of molecular orbital, NO is distinct from CO because it has one unpaired electron in its 2π* orbital. As occupation of an antibonding state usually results in the reduction of binding energies, not surprisingly the binding energy within the NO * To whom correspondence should be addressed. Email: talat@physics. ucf.edu. Phone: 407-823-5785. Fax: 407-823-5112.

molecule (6.53 eV) is smaller than that of CO (11.14 eV).13 Also, upon adsorption on metal or metal oxide surfaces, the 2π* electrons might be given to or taken from the surface. Therefore, it is of great interest to know to what extent molecular orbital structure plays a role in determining the behavior of the two molecules on this ruthenium dioxide surface. Experimental studies of the interaction of NO with RuO2(110)12 have already led to a number of conclusions about the reaction intermediates and products. It would be interesting to see if these results also follow from theoretical calculations. The main focus of the work here is thus to examine through considerations of system energetics possible NO adsorption sites and their binding energies, the propensity of NO to oxidize with neighboring oxygen atoms (O-bridge), and the barrier to its dissociation on RuO2(110). We also provide some insights into the formation of N2O and N2 from NO molecules adsorbed on RuO2(110). Some details of the electronic structure calculations, which are based on density functional theory (DFT), are provided in Section 2. This is followed by a presentation of the results and their discussion in Section 3, and our conclusions are summarized in Section 4. 2. Model Systems and Theoretical Method A stick and ball model for the stoichiometric RuO2(110) surface is shown in Figure 1. In the bulk, Ru atoms have a coordination of six while that of the O atoms is three. On the RuO2(110) surface, however, there are two types of Ru atoms: the six-fold coordinated [Ru-bridge in Figure 1] and the fivefold coordinated [Ru-cus in Figure 1]. This surface has also two types of O atoms: the three-fold coordinated [O-layer in Figure 1] and the two-fold coordinated [O-bridge in Figure 1]. We have carried out nonspin-polarized calculations based on the DFT14 in the generalized gradient approximation (GGA)

10.1021/jp072063a CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007

12362 J. Phys. Chem. C, Vol. 111, No. 33, 2007

Hong et al.

Figure 2. The calculated structural parameters of clean RuO2(110) for a slab with five O-RuO-O layers (showing only the top three layers). Only inequivalent atoms are shown. The values in parentheses are bulk value, and length unit is Å.

3. Results and Discussion

Figure 1. (a) Ball-and-stick model of the stoichiometric RuO2(110) surface in perspective view. Small spheres are O atoms and large spheres are Ru atoms. (b) The same surface in the top view where the letters A-C list adsorption sites chosen for calculations. (A, on top of Ru-cus; B, bridge site; C, on-top of O-bridge.)

using the Perdew-Burke-Ernzerhof functional for the exchangecorrelation energy.15 We have also included spin polarization in selected calculations and find that its effect, while significant for gaseous NO, is not pronounced for NO on RuO2(110). The computer code used was Quantum-Espresso16 where the oneparticle Kohn-Sham equations are solved self-consistently using a plane-wave basis set in the ultrasoft pseudopotential scheme.17 A kinetic energy cutoff of 408 eV was used for the plane-wave basis set. To simulate the influence of NO coverage, two types of surface unit cells were used: (1 × 1) for one monolayer and (2 × 1) for one-half monolayer coverage. Here a monolayer is defined as the configuration in which all Ru-cus sites are fully occupied by the adsorbate. The slab supercell consists of five (in some cases three) O-RuO-O layers of Ru and O atoms, separated by a vacuum thickness of 16 Å. The MonkhorstPack scheme18 was used for k-point sampling of the Brillouin zone. A grid of (6 × 6 × 1) points resulted in 16 irreducible special k-points for (2 × 1 × 5) slab, while a (12 × 6 × 1) grid generated 28 k-points for (1×1×5) slab. A Fermi level smearing of 0.19 eV was applied.19 The NO molecules were adsorbed on both sides of the slab. Because considerations of NO dissociation and the associated reactions require a larger surface unit cell [(2 × 1) or (3 × 1)], such calculations were performed with a slab of three O-RuO-O layers. For these surfaces, the NO molecule was present on one surface with the first O-RuO-O layer fully relaxed while the other surface was held fixed.

3.1. Structure of Clean RuO2(110). In our calculations of bulk RuO2, in the rutile structure a Ru atom is surrounded by four basal and two apical O atoms with bond lengths of 2.04 and 2.01 Å, respectively. The corresponding (110) plane of the bulk exhibits lattice constants of 3.20 Å × 6.56 Å, which are slightly larger (2.9%) than the lattice parameters (3.11 Å × 6.38 Å), found experimentally.20,21 Our structure parameters for clean RuO2(110) calculated with a slab of five O-RuO-O layers is shown in Figure 2. All atoms were fully relaxed, and the relaxation parameters are in agreement with other theoretical calculation.22 Note that bond lengths of Ru-cus with the neighboring O atoms, as well as that of O-bridge with Ru-bridge, are shortened with respect to our calculated bulk values (presented in parenthesis in Figure 2). For example, the bond length between Ru-cus and the O atom below it is reduced from 2.012 Å in the bulk to 1.937 Å, while that between O-bridge and Ru-bridge goes from a bulk value of 2.044 to 1.968 Å. These changes may be attributed to the charge redistribution at the surface induced by the creation of the dangling bonds on top of Ru-cus and O-bridge on RuO2(110).22 For clean RuO2(110), one quantity of interest is the binding energy of O-bridge to Ru-bridge. We find it to be 1.64 eV, with respect to that of gaseous O2, close to that calculated by Kim et al.23 3.2. NO Adsorption Geometry and Ensuing Changes in Surface Electronic Structure. To examine the geometry and the energetics of NO adsorption on the surface, we have calculated the adsorption energy (Ead) for several sites on RuO2(110) shown in Figure 1b and summarized the results in Table 1. The following standard equation relating system total energies is used to calculate Ead:

Ead ) E[NO on RuO2(110)] - E[clean RuO2(110) ] - E[NO] (1) where the total energy of a NO molecule, E[NO], was accurately evaluated in a (10 Å × 10 Å × 10 Å) supercell using spinpolarized calculations, while a 5-trilayer supercell was used to calculate the other two total energies. The preferred adsorption site for 1 ML of NO is on top of Ru-cus (site A in Figure 1b) with an adsorption energy of 1.61 eV (1.72 eV at 0.5 ML coverage) and with the N end of NO bonded to Ru-cus. The next preferred site is on top of O-bridge (site C in Figure 1b) with adsorption energy of 0.82 eV. However, the adsorption

Interaction of NO with RuO2(110) Surface

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12363

TABLE 1: Site Dependent Energetics of NO Adsorption on RuO2(110) from Our Calculationsa adsorption energy

adsorption site A: on-top of Ru-cus B: bridge site C: on-top of O-bridge a

coverage

without zero-point energy

with zero-point energy

1 ML 0.5 ML 1 ML 1 ML 2 ML

1.61 eV 1.72 eV 0.47 eV 0.82 eV 0.74 eV

1.49 eV 1.6 eV 0.38 eV 0.74 eV 0.66 eV

Adsorption energies were calculated for the sites shown in Figure

1b.

energy at the bridge site (B in Figure 1b) is only 0.47 eV. We also considered the adsorption of NO at the above sites (A and C in Figure 1b) with the O end bonded to the surface, but the calculated adsorption energies, being in the range of 0.1-0.2 eV, are much smaller than the ones presented above. As the coverage increases from 0.5 to 1 ML, the adsorption energy of NO decreases by 0.11 eV, (as seen in Table 1,) implying a repulsive interaction among adsorbed NO molecules. In all cases above, the lowest energy configuration is with the NO axis in the upright position. The calculated adsorption energy is larger than the experimental value of 1.34 eV (1.38 eV at low coverage) by about 0.3 eV.12 When including the zero-point energy correction, the adsorption energy for 1 ML becomes 1.49 eV (1.61 eV at 0.5 ML), thereby reducing the discrepancy with experimental data to 0.15 eV (0.23 eV at lower coverage). The above difference of about 0.15 eV-0.23 eV between the calculated and experimental values of the adsorption energy for NO on RuO2(110) is intriguing. This is particularly so because the same is not the case for CO adsorption. Using the same calculational setup as above, we find our calculated adsorption energy of CO on top of Ru-cus on RuO2(110) to be 1.05 eV with zero-point energy correction and 1.18 eV (in agreement with previous theoretical results24,25) without zeropoint energy in excellent agreement with the experimental value of 1.0 eV.26 Because spin polarization plays a significant role in determining the energetics of the NO molecule, it may be argued that the same would be the case for NO adsorbed on RuO2(110) and the accuracy of our nonspin-polarized calculations may be in question. We have thus carried out calculations of NO on RuO2(110), which include spin polarization, to find a change of only 30 meV in the adsorption energy and almost negligible magnetization, indicating that both the clean and NOcovered RuO2(110) surfaces are not spin-polarized. The lack of spin polarization was also found for NO adsorbed on metal surfaces.27,28 In particular, the local spin moment of NO was found to be quenched by the metallic states of Ru(001).27 The discrepancy of 0.2 eV in the experimental and calculated value of Ead may also be attributed to the accuracy of models used for extracting desorption/adsorption energy from experimental data. Note that the above experimental value of 1.34 eV (1.38 eV at low coverage) was obtained from temperature desorption spectroscopy (TDS) data using the Redhead equation,29 which is based on simple assumptions and may not provide exact results. Still, it is interesting to note that our calculated CO adsorption energy is in very good agreement with that in ref 26 in which the experimental adsorption energy was also obtained using the Redhead equation. Of course, there is always the possibility that for NO on RuO2(110) we have another example of overestimation of binding energy by DFT

Figure 3. The calculated structural parameters of NO/RuO2(110) for (a) 0.5 ML coverage and (b) 1 ML coverage. For (a), the structural parameters for the NO-bonded Ru-cus are shown. The values in parentheses are bulk ones, and length unit is Å.

calculations, which are based on current GGA functionals.28 Only future improvements to these functionals would settle the issue. Before leaving the subject of adsorption energies, we would like to point out that the calculated adsorption energies for both NO and CO on this oxide surface are smaller than that found on Ru(001). For example, for NO on Ru(001) in DFT calculations the adsorption energy lies in the range of 2.1-2.6 eV (ontop site),28 higher than what we report here. In the case of CO also, the calculated adsorption energy of 1.18 eV is smaller than that on Ru(001) by 0.5 eV.30 This reduction of adsorption energy of NO or CO on RuO2(110) may be related to the existence of the electronegative O atoms strongly bonded to Ru-cus atoms at RuO2(110). Below we provide detailed information on the characteristics of NO adsorbed on the two types of sites on RuO2(110). 3.2.1. Geometric and Electronic Structure of NO on Ru-cus. We first examine in detail the geometric structure of NO on top of Ru-cus. In Figure 3, the calculated bond lengths are shown for 0.5 ML (Figure 3a) and 1 ML coverage (Figure 3b). Note that in Figure 3a only part of the structural parameters is shown, namely, only for NO-Ru-cus, and not for the other unoccupied Ru-cus atoms. The N-Ru-cus bond length increases from 1.839 Å at 0.5 ML to 1.848 Å at 1 ML. This increase is correlated to higher adsorption energy of NO (i.e., stronger bond between NO and Ru-cus) at low coverage, as seen earlier. The NO intramolecular bond length is found to increase slightly from 1.161 Å at 0.5 ML to 1.164 Å at 1 ML but still remains shorter than the N-O bond length (1.174 Å). As we will see later, this reduction in bond length is related to the reduction in the occupation of 2π* antibonding state of NO from gas phase to the adsorbate state. On the other hand, from the structural

12364 J. Phys. Chem. C, Vol. 111, No. 33, 2007 parameters in Figures 2 and 3a for which the structural parameters for the NO-bonded Ru-cus are shown, we find that at low NO coverage the bond length of Ru-cus with both of its neighboring atoms increases; Ru-cus-O-layer bond increases from 2.033 to 2.089 Å and Ru-cus-O-layer bond changes from 1.937 to 2.068 Å. Similar increments are also found for the bond between the O-bridge and Ru-bridge atoms. However, as NO coverage is increased to 1 ML (Figure 3b) the abovementioned bonds of Ru-cus decrease slightly, while the bond length of O-bridge with Ru-bridge increases slightly. These structural changes are consistent with the charge redistribution between the bonds at the surface, induced by NO adsorption on top of Ru-cus. At low coverage, the N and Ru-cus atoms attract charges from atoms around them, and at high coverage the Ru-cus, which was previously occupied by NO, slightly releases charges to the surrounding atoms while the previously empty Ru-cus attracts. A similar charge redistribution has been observed for the OH-bridge formation on RuO2(110) by Sun et al.22 and has been called a bond-order propagation effect. In the absence of any direct experimental structural data for NO on RuO2(110), we turn to results for NO on Ru(001) to get some insight into the calculated surface bond length. From lowenergy electron diffraction measurements, Stichler and Menzel31 find the N-Ru bond length for on-top site adsorption to lie at 1.72 ( 0.04 Å, while that of N-O is 1.13 ( 0.06 Å. At the same time using near edge extended X-ray absorption fine structure, Esch et al. report the N-O bond to be 1.20 ( 0.01 Å.32 The point here is that our calculated N-Ru-cus bond length (1.839-1.848 Å) for NO on top of Ru-cus on RuO2(110) is larger than what was found experimentally for NO on metallic Ru(001). This difference is consistent with the difference in the adsorption energy of NO on the metal and the metal oxide surface, as mentioned earlier. We now turn to an analysis of changes in the surface electronic structure on NO adsorption, as elaborated in Figure 4 through a plot of the charge density differences calculated using the equation

∆F ) F[NO/RuO2(110)] - F[RuO2(110)] - F[NO] (2) where F[NO/RuO2(110)] and F[RuO2(110)] represent the charge density of RuO2(110) with and without NO. An advantage of the charge difference plot is that we can visualize charge density changes before and after adsorption. For comparative purposes, we have also calculated the charge difference plot for the wellstudied case of CO on RuO2(110) in Figure 4a. As is well known and noted from Figure 4a, charge accumulates in the region between C and Ru-cus, and the resulting charge distribution clearly characterizes the formation of a covalent bond between them. A bonding state between C and Ru-cus, as well as the 2π* antibonding state of C and O (note “+” charge redistribution near C and O) in Figure 4a, are commonly attributed in the literature33 to charge donation (from 5σ state of CO to 4d states of Ru-cus) and back-donation (from 4d states of Ru-cus to 2π* state of CO). A similar bonding mechanism is found for NO on RuO2(110) as shown in Figure 4b. We note the development of a bonding state between N and Ru-cus, as well as the development of 2π* antibonding state of NO similar to those of CO. However, many features substantially different from those of CO are noticeable. First of all, there is a much larger charge accumulation between N and Ru-cus. In particular, the charge accumulation around the N atom is remarkably high, as well as unique in shape. Second, although a charge accumulation in the 2π* state of NO is seen in the [110] × [11h0] plane, only a charge depletion occurs on the O end on the [110]

Hong et al.

Figure 4. A plot of charge density difference: (a) for CO and (b) for NO adsorbed on Ru-cus with the upright axis at 1 ML coverage. Charge accumulation and depletion regions are shown as plus (+) and minus (-), respectively.

× [001] plane (Figure 4b). Third, much larger charge depletion is found on the top and the bottom regions of Ru-cus (perpendicular to the surface), while charge accumulation occurs on the sides of Ru-cus, parallel to the surface. This latter accumulation on the sides of Ru-cus is responsible for the stiffening of Ru-cus-O-layer bond at high coverage. The larger charge accumulation in the region between N and Ru-cus as compared to that between C and Ru-cus represents the strength of the (covalent) bond between them and explains the 0.4 eV difference in the adsorption energy between CO and NO on RuO2(110). The source of this accumulated charge in the region just below the N atom in the [110] × [11h0] plane may be found with help of the plot of the local density of states of NO/RuO2(110) shown in Figure 5. We find that the density of state of the 2π* state of NO shifts to beyond the Fermi energy with a minor occupation of its tail at the Fermi energy (bottom panel), while the density of states of Ru-cus at the Fermi energy reduces significantly upon NO adsorption (compare second and third panels). The electron depleted from the 2π* state may find its way to the substrate or to the N atom. If it is transferred to the substrate, it may stiffen the Ru-cus bonds with neighboring atoms. However, the structural changes induced by NO adsorption on Ru-cus, as we have seen, are contrary to this prediction, and signify the opposite, that is, charge transfer takes place from

Interaction of NO with RuO2(110) Surface

Figure 5. Local density of states (LDOS) for NO/RuO2(110). This LDOS graph corresponds to the charge density in Figure 4b.

the substrate to NO. These findings lead to the conclusion that the accumulated charge around the N atom comes from the 2π* state of NO as well as from d states of Ru-cus. This contribution of 2π* electron in the bonding of N and Ru-cus is thus the rationale for why the NO-Ru-cus bond is stronger than the CO-Ru-cus one on RuO2(110). This difference in the bonding of CO and NO to RuO2(110) has further consequences for the reactivity of these adsorbed gases on this oxide surface, as we shall see. Before leaving the subject of the structure of NO on RuO2(110), we need to address the possibility of a tilt in the NO axis as has been suggested in TDS experiment.12 In this regard, it is interesting to note the coexistence of the two adsorption states of NO on RuO2(110) at saturation coverage in TDS experiment such that the molecular axis of one-third of the NO atoms are slightly tilted while that of two-thirds of them are reported to be in the upright position with respect to the surface normal.12 This is an interesting scenario and suggests an accommodation of two types of NO geometry at saturation coverages for surface stability. For NO on Ru(001), the existence of the opposite surface dipole moments (positive one for NO adsorbed in terminal configuration and negative one for NO adsorbed in three-folded configurations) is considered as a stability condition of the adsorbed surface.34,35 If NO on RuO2(110) was to possess dipole moment, with increasing coverage the net dipole moment would increase. The advent of a new adsorption state at saturation coverages (i.e., slight axis tilt)12 could be a means of reducing the accumulated surface dipole moment. Keeping this in mind, we have calculated the adsorption energy for NO with a tilted axis at 1 ML. Note that the choice of the supercell in our calculations forces the axis of all NO molecules to be either upright or tilted. We find the upright axis to be preferred just by 15 meV from a tilted one at 7°. The small difference in the adsorption energy between the upright and tilted adsorption states suggests that the axis of NO could be tilted at saturation coverage at finite (nonzero) temperatures as in experiments. 3.2.2. Electronic and Geometric Structure of NO on O-bridge. As already mentioned, the adsorption of NO on O-bridge is also interesting. It is the secondary adsorption site from Table 1 (C in Figure 1b), and it is the one reported from experiments12

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12365

Figure 6. Formation of NO2 on top of O-bridge. (a) The geometry and the charge density distribution. Note that the Ru-bridge atom is out of the plane. (b) A plot of charge density difference for the NO2 on Ru-bridge shown in (a) in which charge depletion and accumulation region are shown as minus (-) and plus (+), respectively.

for coverages higher than 1 ML (according to the earlier definition) with a desorption energy of 0.66 eV. As seen from Table 1, our calculated adsorption energy for 2 ML (i.e., with all Ru-cus and O-bridge sites occupied by NO) is also 0.66 eV (0.74 eV without zero-point energy correction), giving us perfect agreement with the experimental value! It has also been suggested from analysis of experimental data12 that the N end is bonded to the O-bridge atom with a tilted N-O axis. Our calculated results for the adsorption geometry (Figure 6a) indeed show this to be the case. However, the N atom is not exactly on top of the O-bridge atom; rather, it is shifted by 0.56 Å, mainly along the [11h0] direction. In other words, both the N-O axis and the N-O-bridge axis are tilted. The angle O-bridgeN-O is 111°, while the angle N-O-bridge-Ru-bridge is 118°. The optimized structure reveals that the interatomic bond lengths N-O, N-O-bridge, and O-bridge-Ru-bridge are 1.18, 1.46, and 2.17 Å, respectively. For comparison, our calculated structure of NO2 in the gas phase has a N-O bond length of 1.21 Å and the angle O-N-O is 133°, which is in excellent agreement with experimental results,36 to be compared with 111° for O-N‚‚‚O-bridge. The charge density distributions in Figure 6a,b confirm that this newly formed O-N‚‚‚O-bridge complex resembles NO2 but with one substantially elongated arm: the N‚‚‚O-bridge bond (1.46 Å) whose stretching can be readily rationalized. We already know that the O-bridge-Ru-bridge bond energy is about 1.63 eV, to be compared to the adsorption energy of 0.66 eV for NO. The larger strength of the O-bridgeRu-bridge bond helps stretch the N-O-bridge bond such that it is not equivalent to the N-O bond (1.18 Å) in the structure presented in Figure 6. These findings are also in accordance with experiments in which NO is found to desorb from the Rubridge site (after breaking the bond with O-bridge) instead of NO2, upon heating. 3.3. NO Oxidation. Experimentally, NO is not found to oxidize to NO2 on RuO2(110), neither through desorption of the O‚‚‚N-O surface complex as discussed above, nor via the interaction of NO with O-bridge. On the other hand, CO oxidizes readily to CO2 on RuO2(110), even at room temperature.6,7

12366 J. Phys. Chem. C, Vol. 111, No. 33, 2007

Hong et al.

Figure 8. NO dissociation. (a) Schematic model of the calculated transition state and (b) the energy barrier are shown.

Figure 7. The calculated relevant energies for NO and CO oxidation process on RuO2(110): (a) the total energy change and the activation energies for (b) NO and (c) CO.

Related DFT calculations37 have also corroborated the experimental results for CO oxidation, and even the experimental rate of CO2 formation38 could be reproduced using an ab initio kinetic Monte Carlo approach.39 To provide an understanding for the different behavior between CO and NO toward oxidation on RuO2(110), we first consider the energetics of the involved processes. The calculated relevant energies are summarized in Figure 7. The basic assumption in Figure 7a is that NO or CO first adsorbs on the stoichiometric RuO2(110) [s-RuO2(110)], then reacts with the O-bridge atom on the surface, and eventually desorbs as CO2 or NO2 with an O-bridge vacancy left on the surface [r-RuO2(110)]. The energetics in Figure 7 indicate that to go from NO adsorbed on RuO2(110) to the final state of NO2 (gas phase) and r-RuO2(110) requires an energy of 2.08 eV, while CO2 formation is exothermic. Simply put, NO oxidation is energetically far more costly than that of CO on RuO2(110). Further insights into the differences in the behavior of CO and NO on RuO2(110) are obtained by comparing the calculated energy barriers, which are summarized in Figure 7b,c. Note that for both NO (Figure 7b) and CO (Figure 7c), the activation energy for oxidation is about 1.22 eV (1.25 eV).24 The locations of the final states are, however, remarkably contrasting. The final state for NO oxidation is just 0.37 eV below the transition state and still 0.85 eV higher than initial state, while that for CO oxidation is far below the transition state and even lower than initial state. Basically this means that NO2 reduction process

(right-to-left process in Figure 7b) is much more probable than the NO oxidation process (left-to-right process in Figure 7b), while for CO the reverse is the case. Furthermore, the desorption energy level of NO2 (Figure 7b) is situated at 1.23 eV above the final state while that of CO2 (Figure 7c) is located at 1.6 eV below the transition state. These theoretical considerations thus lead to the conclusion that while CO2 desorption can occur immediately after the transition state is reached, NO 2 formation is energetically unfavorable. 3.4. NO Dissociation. As we will see in the next section, NO dissociation probability on RuO2(110) is of significance in elucidating its reaction paths on this surface. In this section, our focus is on the calculation of this particular energy barrier. In principle, one should carry out multidimensional calculations of the energy landscape, which would be an insurmountable task. Guided by experimental insights, we have confined our calculations to the [001] direction along which the adjacent NO molecules, adsorbed on Ru-cus sites that are separated by 3.20 Å, are expected to be the primary reactants. We calculate the total energy of the system by fixing the position of the O atom at a certain point along the [001] direction and by gradually increasing the bond length to 3.20 Å, thereby treating the N-O bond length as the reaction variable. The calculations are repeated for various positions of the O atom until it finally reaches the next Ru-cus atom. The results are shown in Figure 8. A transition state with the activation energy of 3.22 eV is found in which the N and O atoms are separated by 2.06 Å. The obtained activation energy of 3.22 eV is larger than the adsorption energy of NO (1.72 eV) confirming the observation that NO dissociation is not evidenced on RuO2(110).12 The calculated activation energy is also significantly larger than that on Ru surfaces. For example, the predicted activation energy for NO dissociation on the Ru(0001) surface was 1.28 eV.27 3.5. Formation of N2O and N2. In the experiments of Wang et al.,12 a small amount of N2O was observed to be produced through the reaction of two adjacent NO on RuO2(110) at high coverages (g0.66 ML), while N2 desorption was observed at 180 K from the very early state of NO adsorption (i.e., very low coverage at 85 K) and was considered to be formed by the thermal decomposition of N2O. Although there was clear evidence for N2 desorption, no strong signal for the frequency

Interaction of NO with RuO2(110) Surface

Figure 9. (a) Schematic model of the intermediate products and the total energy change for N2O formation. Desorption level is marked as a dotted line. (b) Schematic model of the calculated geometry of N2O formed on RuO2(110).

of the N-N vibrational mode, V(N-N), was observed in the electron energy loss spectroscopy measurements. Two reaction pathways, which lead to N2O formation on RuO2(110), were proposed:12

NO + NO f 2NOad f NOad + Nad + Oad f N2Oad + Oad (3) NO + NO f (NO)2 f N2O + Oad f N2 v + 2Oad (4) As discussed in Section 3.4 above, our calculations find a large barrier for NO dissociation on RuO2(110) thereby not supporting the feasibility of first pathway (3). To comment on the viability of the other proposed pathway (4), we need to examine the energetics of formation of N2O as an intermediate that then gives rise to N2. Figure 9a shows schematically the intermediate products leading to N2O and eventually to N2 formation with a corresponding total energy change as reaction (4) proceeds. Let us concentrate first on the last stage of the reaction. As we see from Figure 9a, the total energy of N2 + 2O is slightly lower (by 40 meV) than that of its precedent, N2O + O, while its desorption energy level is equal to the total energy of N2O + O. The latter implies that once thermal decomposition of N2O into N2 and O occurs, the newly produced N2 should immediately desorb, in agreement with experimental findings. Figure 9b shows our calculated structure for N2O formed on RuO2(110) from two adjacent NO’s. This structure in which the N-N bond is at a very small angle (27°) with respect to the surface is in agreement with experimental indications.12 Now let us turn to the initial stage of the reaction, that is, NO + NO f N2O + Oad. From consideration of total energy, 1.35 eV is needed for the formation of N2O from two adjacent NO on RuO2(110). Note that in the above consideration we have bypassed the formation of the intermediate complex (NO)2. In this work, we provide calculation of the upper limit to the barrier for (NO)2 formation. The energy barrier for this process (or equally N2O or N2 formation energy) is defined to be the difference in total energy between that of (NO)2 and that of

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12367 NO + NO. Our calculated energy barrier is 3.1 eV, which is far greater than the experimental value.12 Our method used for the calculations of the energy barrier can search only a portion of the multidimensional phase space, while the exact energy barrier should be that of the minimum energy path in the entire phase space. Nonetheless, our calculated total energy change is a good indicator of relative system energetics, and we take 1.35 eV as the lower bound for the energy barrier. This estimated minimum value is, however, larger than the experimental value. We leave more detailed calculations for the future as the feasibility of this reaction is not the main point of this work. We also have some concern about the viability of reaction pathway (4) at coverages less than 1/2 ML and at low temperature in connection with what we know from experiments about the NO coverage dependence of the reaction products. For N2O and N2 to be produced at low coverage and at low temperature (180 K), it is essential that NO be able to diffuse with a small energy barrier on RuO2(110). Without indulging in a detailed calculation of the transition path, a quick estimate yields a diffusion barrier of 1.4 eV (from the difference in the total energy of the system with NO at site A and B in Figure 1), which is quite large for diffusion to occur even at room temperature. Basically, this means that NO sticks to Ru-cus and hardly diffuses once it is adsorbed on top of Ru-cus. Hence, for reaction (4) to occur, two NO must be adsorbed nearby in adjacent Ru-cus sites, which is feasible when the coverage is more than 1/2 ML. At such coverages (preferably above 2/3 ML) only, N2O desorption was observed.12 The question of N2 formation at all coverages is still unanswered. It is likely that there is another low-coverage, low-temperature pathway for N2 formation on RuO2(110). Such a pathway may occur at defects present at the surface in the experiment. We leave investigation of search for such a pathway to future study. 4. Conclusion In this study, we have carried out nonspin-polarized (and spinpolarized when needed) calculations based on the DFT in the GGA using the Perdew-Burke-Ernzerhof functional for exchange-correlation energies to investigate the adsorption and dissociation of NO on RuO2(110) surface. NO adsorbs preferably on top of Ru-cus with an upright axis, and the adsorption energy is understandably coverage dependent. Once all Ru-cus sites are occupied, NO adsorbs in the O-bridge site to form an O-N‚‚‚O-bridge complex with the adsorption energy of 0.66 eV. Comparatively speaking, we find the unpaired 2π* electron on NO to contribute to the bonding causing NO-Ru-cus bond to be stronger than the CO-Ru-cus. The activation energy for NO oxidation on RuO2(110) is found to be about the same as that for CO oxidation. However, the locations of the final states in the two cases are remarkably different. For CO oxidation is energetically favorable; for NO oxidation is only transient. Also, the calculated energy barrier for NO dissociation is found to be significantly larger than that for metallic Ru surfaces. Our calculated total energy difference for the two sides of the reaction NO + NO f N2O + O on RuO2(110) may qualitatively explain the observed low rate of N2O production on this surface. Acknowledgment. This work was supported in part by grants from DOE (DE-FG02-07ER15842) and NSF (CHE0548632). Computational resources were provided by NSF Cyberinfrastructure and TeraGrid Grant DMR060035N. T.S.R. also acknowledges the support of the Alexander von Humboldt Foundation and thanks her colleagues at the Fritz-Haber-Institut,

12368 J. Phys. Chem. C, Vol. 111, No. 33, 2007 Berlin for their warm hospitality. We are grateful to Sergey Stolbov and Abdelkader Kara for helpful discussions. References and Notes (1) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 2578 and references therein. (2) Li, G.; Kaneko, K.; Ozeki, S. Langmuir 1997, 13, 5894. (3) Wilde, M.; Seiferth, O.; Al-Shamery, K.; Freund, H.-J. J. Chem. Phys. 1999, 111, 1158. (4) Bender, M.; Seiferth, O.; Carley, A. F.; Chambers, A.; Freund, H.J.; Roberts, M. W. Surf. Sci. 2002, 513, 221. (5) Kim, C. M.; Yi, C. W.; Min, B. K.; Santra, A. K.; Goodman, D. W. Langmuir 2002, 18, 5651. (6) Wang, J.; Fan, C. Y.; Jacobi, K.; Ertl, G. Surf. Sci. 2001, 481, 113. (7) Fan, C. Y.; Wang, J.; Jacobi, K.; Ertl, G. J. Chem. Phys. 2001, 114, 10058. (8) Stampfl, C.; Schwegmann, S.; Over, H.; Scheffler, M.; Ertl, G. Phys. ReV. Lett. 1996, 77, 3371. (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) Bo¨ttcher, A.; Niehus, H.; Schwegmann, S.; Over, H.; Ertl, G. J. Phys. Chem. B 1997, 101, 11185. (11) Bo¨ttcher, A.; Conrad, H.; Niehus, H. Surf. Sci. 2000, 452, 125. (12) Wang, Y.; Jacobi, K.; Ertl, G. J. Phys. Chem. B 2003, 107, 13918. (13) Lide, D. R. CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1997. (14) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133. (15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (16) Baroni, S.; Dal Corso, A.; de Gironcoli, S.; Giannozzi, P. PWscf. http://www.pwscf.org, accessed July, 2006. (17) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (18) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (19) Methfessel, M.; Paxton, A. Phys. ReV. B 1989, 40, 3616.

Hong et al. (20) Boman, C. E. Acta Chem. Scand. 1970, 24, 116. (21) Huang, Y. S.; Park, H. L.; Plooak, F. H. Mater. Res. Bull. 1982, 17, 241. (22) Sun, Q.; Reuter, K.; Scheffler, M. Phys. ReV. B 2004, 70, 235402. (23) Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Wang, J.; Fan, C.; Jacobi, K.; Over, H.; Ertl, G. J. Phys. Chem. B 2001, 105, 3752. (24) Reuter, K.; Scheffler, M. Phys. ReV. B 2003, 68, 045407. (25) Seitsonen, A. P.; Kim, Y. D.; Knapp, M.; Wendt, S.; Over, H. Phys. ReV. B 2001, 65, 035413. (26) Kim, S. H.; Paulus, U. A.; Wang, Y.; Wintterlin, J.; Jacobi, K.; Ertl, G. J. Chem. Phys. 2003, 119, 9729. (27) Hammer, B. Surf. Sci. 2000, 459, 323; Phys. ReV. Lett. 1999, 83, 3681. (28) Gajdoˆs, M.; Hafner, J.; Eichler, A. J. Phys.: Condens. Matter 2006, 18, 13. (29) Redhead, P. A. Vacuum 1962, 12, 203. (30) Gajdoˆs, M.; Eichler, A.; Hafner, J. J. Phys.: Condens. Matter 2004, 16, 1141. (31) Stichler, M.; Menzel, D. Surf. Sci. 1997, 391, 47. (32) Esch, F.; Ladas, S.; Kennou, S.; Siokou, A.; Imbihl, R. Surf. Sci. 1996, 355, L253. (33) Kim, Y. D.; Seitsonen, A. P.; Over, H. Phys. ReV. B 2001, 63, 115419. (34) Umbach, E.; Kulkarni, S.; Feulner, P.; Menzel, D. Surf. Sci. 1979, 88, 65. (35) Feulner, P.; Kulkarni, S.; Umbach, E.; Menzel, D. Surf. Sci. 1980, 99, 489. (36) Classen, S.; Donohue, J.; Shoemaker, V. J. Chem. Phys. 1948, 16, 207. (37) Reuter, K.; Scheffler, M. Phys. ReV. Lett. 2003, 90, 046103. (38) Wang, J.; Fan, C. Y.; Jacobi, K.; Ertl, G. J. Phys. Chem. B 2002, 106, 3422. (39) Reuter, K.; Frenkel, D.; Scheffler, M. Phys. ReV. Lett. 2004, 93, 116105.