Density Functional Theory Study of the Oxidation of Ammonia on

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei, 106, Taiwan, and Instit...
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J. Phys. Chem. C 2009, 113, 17411–17417

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Density Functional Theory Study of the Oxidation of Ammonia on RuO2(110) Surface Chia-Ching Wang,† Ya-Jen Yang,† Jyh-Chiang Jiang,*,† Dah-Shyang Tsai,† and Horng-Ming Hsieh‡ Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, 43, Keelung Road, Section 4, Taipei, 106, Taiwan, and Institute of Nuclear Energy Research, No. 1000, Wenhua Road, Longtan, Taoyuan, 325, Taiwan ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: August 22, 2009

We have used density functional theory (DFT) calculations to investigate the oxidation of ammonia (NH3) on a RuO2(110) surface. We characterized the possible reaction pathways for the dehydrogenation of NHx species (x ) 1-3) and the formation of the oxidation products N2, NO, and H2O. The presence of oxygen atoms on coordinatively unsaturated sites (Ocus) promoted the oxidation of NH3 on the surface. The oxidation of NH3 is possible on both stoichiometric and oxygen-rich RuO2(110) surfaces; in the absence of Ocus (stoichiometric surface), however, NH3 molecules prefer desorption over oxidation. Moreover, the Ocus atoms are the major oxidants in this process; the formations of H2O and NO from bridge oxygen atoms (Obr) are both unfavorable reactions. According to our energetic analysis, in the NHx dehydrogenation pathways, H atom migration from NH2-cus to Obr has the highest barrier by 0.86 eV; it is much lower than the interaction energy of NH3 on the RuO2(110) surface. In terms of nitrogen-atom-containing products, NO, N2, and N2O are all possible products of the oxidation of NH3. The formation of the gaseous oxidation products H2O and NO is determined by their binding energies, whereas that of N2 is controlled by the diffusion of Ncus atoms on the surface. In addition, the selectivity toward the nitrogen-atom-containing products N2 and NO is dominated by the coverage of Ocus atoms on the surface; a higher coverage of Ocus atoms results in greater production of NO. 1. Introduction The decomposition of ammonia (NH3) is an important industrial reaction. The oxidation of NH3 leading to N2 and H2O is of increasing interest in connection with the removal of NH3 from waste streams.1 In addition, the catalytic oxidation of NH3 to NOsthe so-called Ostwald processsis a key step in the production of nitric acid. Other processes, such as the decomposition of NH3 to produce H2 for fuel cells,2-5 the oxidation of NH3 during nitric acid production,6 and the reaction of NH3 with methane to produce HCN,7 are also of high practical importance. The most critical factor in determining the reactivity and selectivity of NH3 decomposition and oxidation reactions is the nature of the catalyst. In recent density functional theory (DFT) studies, several materials have been investigated as catalysts for NH3 dissociation reactions, including Pt,8-10 Rh,8,11,12 Pd,8 and Au.13 In 2005, Wang et al.14 investigated the selective oxidation of NH3 to either N2 or NO on RuO2(110) single-crystal surfaces. They found that the concentration of oxygen atoms adsorbed on these Rucus sites determined both the reactivity and selectivity of the NH3 oxidation process. A 100% selectivity for NO formation was obtained on RuO2(110) at 530 K, a much lower temperature than those applied in typical Ostwald process using Pt-based catalysts (>1100 K). Figure 1 presents a schematic representation of the stoichiometric RuO2(110) surface, which presents exposed rows of 2-fold coordinated oxygen atoms (Obr), 5-fold coordinated Ru atoms (Rucus; cus ) coordinatively unsaturated site), and triply coordinated layer oxygen atoms (O3f), all along the [001] * To whom correspondence should be addressed. E-mail: jcjiang@ mail.ntust.edu.tw. Phone: +886-2-27376653. Fax: +886-2-27376644. † National Taiwan University of Science and Technology. ‡ Institute of Nuclear Energy Research.

Figure 1. Ball-and-stick model of the stoichiometric RuO2(110) surface.

direction. Additional oxygen atoms may be adsorbed on top of the Rucus atoms (so-called Ocus) upon further exposure to O2. The Rucus atoms on the surface behave as catalytically active sites onto which NH3 or O2 molecules can be adsorbed from the gas phase. Investigations of catalytic reactions performed on RuO2 surfacesssuch as CO oxidation15-19 and HCl oxidation (the Deacon process)20sreveal the high catalytic activity of the RuO2(110) surface. In a previous study,21 we modeled the binding of NHx (x ) 1-3) species on stoichiometric and oxygen-rich RuO2(110) surfaces. We characterized the detailed interactions between the NHx species and the surfaces by analyzing the density of states (DOS), the electron density differences, and the vibrational frequencies. In this study, we further investigated the oxidation of NH3 on stoichiometric and oxygen-rich RuO2(110) surfaces. We used DFT calculations to predict the mechanisms for the decomposition of NHx species and for the formation of H2O, N2, NO, and N2O, and used energetic analyses to propose possible reaction pathways for the oxidation of NH3 on the RuO2(110) surface. Our DFT calculations suggested reaction mechanisms for the oxidation of NH3 on the RuO2(110) surface

10.1021/jp905627k CCC: $40.75  2009 American Chemical Society Published on Web 09/14/2009

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that agreed well with the experimental observations reported by Wang et al.14 2. Computational Details All DFT calculations were performed using the Vienna ab Initio Simulation Package (VASP).22-26 The generalized gradient approximation (GGA) was used with the functional described by Perdew and Wang27 and a cutoff energy of 300 eV. Spinpolarization calculations were performed for all of the structural optimizations, and electron-ion interactions were investigated using the projector augmented wave method.28 The RuO2(110) surface was modeled as a two-dimensional slab in a threedimensional periodic cell. The slab was a 2×1 surface having the thickness of four O-Ru-O repeat units, which is equivalent to 12 atomic layers. The five uppermost atomic layers were relaxed in all structural optimizations. A 12.8 Å vacuum space was introduced to curtail interactions between the slabs. For this (2×1)-RuO2(110) surface model, the k-points of 3×3×1 were set by Monkhorst-Pack. More details of the surface model and the benchmark of different cutoff energy and k-points settings are available in the Supporting Information. For transition state (TS) determination, the nudged elastic band (NEB) method, implemented in VASP, was applied. The validity of all the optimized structures and the determined TSs were checked through normal-mode frequency analysis. For a real minimum on a potential energy surface, all frequencies must be positive; a TS must have one imaginary frequency corresponding to the reaction coordinate. In the vibrational frequency calculations, the upper two atomic layers and adsorbates were relaxed; all others were fixed. The estimated interaction energy (Eint) was calculated using the formula

Eint ) EA/surf - (Esurf + EA) where Esurf is the energy of the stoichiometric surface or the oxygen-rich surface, EA is the energy of a single adsorbate, and EA/surf is the total energy of the adsorbate on the surface. A negative value of Eint indicates an exothermic adsorption or chemisorption process. 3. Results and Discussion A. NHx Dehydrogenation. We determined the possible reaction pathways for the dehydrogenation of NHx on the RuO2(110) surface that proceeded through reactions with surface oxygen or hydroxyl species. In a previous study, we found that NH3 adsorbs initially on Rucus sites.21 Figure 2 illustrates our previously determined binding geometries for NHx species on stoichiometric and oxygen-rich RuO2(110) surfaces.21 The dehydrogenation process involves hydrogen atom migration from NHx to O or OH on either the Rucus site (Ocus and OHcus) or bridge site (Obr and OHbr), thereby producing OH or H2O on the surface. The reaction pathways for the dehydrogenation of NHx species can be summarized as follows: NH3 dehydrogenation

NH3-Ocus NH3-OHcus NH3-Obr NH3-OHbr

NH3-cus NH3-cus NH3-cus NH3-cus

+ + + +

Ocus f NH2-cus + OHcus OHcus f NH2-cus + H2Ocus OHcus f NH2-cus + H2Ocus OHbr f NH2-cus + H2Obr

Figure 2. Top views of NHx species adsorbed onto RuO2(110) surfaces: (a-c) on the stoichiometric surface (θ ) 1/2 ML); (d-g) on the oxygenrich surface.20

TABLE 1: Barriers (E‡, eV), Reaction Energies (∆E, eV), and Imaginary Frequencies (IMF, cm-1) for the NH3 Dehydrogenation Pathways on the RuO2(110) Surface pathway

E‡

∆E

IMF

NH3-Ocus NH3-Obr NH2-Ocus NH2-OHcus NH2-Obr NH2-Obr-2 NH-Ocus NH-OHcus NH-Obr NH-OHbr

0.71 0.44 0.48 0.31 0.86 0.59 ∼0 ∼0 ∼0 0.48

0.62 0.43 0.34 0.31 0.11 0.43 -0.82 -0.93 -0.78 0.47

i157 i218 i897 i679 i255 i689

i196

NH2 dehydrogenation

NH2-Ocus NH2-OHcus NH2-Obr NH2-OHbr

NH2-cus NH2-cus NH2-cus NH2-cus

+ + + +

Ocus f NHcus + OHcus OHcus f NHcus + H2Ocus Obr f NHcus + OHbr OHbr f NHcus + H2Obr

+ + + +

Ocus f Ncus + OHcus OHcus f Ncus + H2Ocus Obr f Ncus + OHbr OHbr f Ncus + H2Obr

NH dehydrogenation

NH-Ocus NH-OHcus NH-Obr NH-OHbr

NHcus NHcus NHcus NHcus

Table 1 lists the barriers, reaction energies, and imaginary frequencies of the TSs; Figure 3 displays the TS geometries of the NHx dehydrogenation pathways. NH3-cus Dehydrogenation. There are four possible NH3-cus dehydrogenation pathways, but only two of them are feasible. According to our calculations, the final states of pathways NH3-OHcus and NH3-OHbr are unstable; the minima of NH2-cus + H2Ocus and NH2-cus + H2Obr do not exist. The H atoms on these two formed H2O molecules migrated back to NH2-cus after geometry optimization (i.e., the dehydrogenation of NH3-cus will not follow the NH3-OHcus and NH3-OHbr reaction pathways); this phenomenon might due to the higher proton

Oxidation of Ammonia on RuO2(110) Surface

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Figure 3. Top views of the TSs for NHx dehydrogenation on the RuO2(110) surface: (a) NH3-Ocus, (b) NH3-Obr, (c) NH2-Obr, (d) NH2-Ocus, (e) NH2-OHcus, (f) NH2-Obr-2, and (g) NH-OHbr.

affinity of NH2-cus, relative to OH species on the RuO2(110) surface. For the other two NH3-cus dehydrogenation pathways, the reaction barriers are 0.71 eV (NH3-Ocus) and 0.44 eV (NH3-Obr). The imaginary frequencies at the TSs of these two pathways are 157 cm-1 (NH3-Ocus) and 218 cm-1 (NH3-Obr). These relatively low imaginary frequencies at the TSs arose because their vibration modes are not pure N-H stretches; they are also coupled with the libration mode of NH3-cus. In addition, the TSs for the NH3-cus dehydrogenation pathways are late TSs: the value of d(O-H) of 1.02 Å in the TS of NH3-Ocus is close to the value of 0.98 Å in the final state; in pathway NH3-Obr, the value of d(O-H) at the TS is 1.08 Å, also close to the value in the final state (1.02 Å). Additional selected geometrical parameters are listed in the Supporting Information. The reaction energies of the two pathways NH3-Ocus and NH3-Obr are endothermic: 0.62 and 0.43 eV, respectively. Because both reactions have late TSs, the reaction energies are close to the reaction barriers. In particular, the reaction energy for NH3-Obr is almost as high as the TS energy (0.44 eV). On the stoichiometric RuO2(110) surface, Obr atoms are the only oxidants available for the oxidation of NH3. Therefore, initialization of the NH3-cus dehydrogenation process should follow pathway NH3-Obr. If the surface features Ocus atoms, as in the oxygen-rich RuO2(110) surface, then dehydrogenation could follow the NH3-Ocus pathway. NH2-cus Dehydrogenation. On the stoichiometric RuO2(110) surface, because no Ocus atoms exist on the surface, the possible dehydrogenation of NH2-cus involves H atom migration to either Obr (NH2-Obr) or OHbr (NH2-OHbr). Pathway NH2-OHbr is, however, not feasible because the final state, NHcus + H2Obr, is not stable; no minimum can be found on this structure. In NH2-Obr, the reaction barrier is 0.86 eV and the reaction energy is 0.11 eV; Figure 3c presents a top view of the TS structure. This pathway has the highest reaction barrier of all the NHx dehydrogenation processes; during the initial stages, the N-H bond of NH2-cus does not point at the Obr atom and the additional molecular rotation of NH2-cus required for dehydrogenation results in the higher reaction barrier. When Ocus atoms are present on the RuO2(110) surface (oxygen-rich surface), the barriers of NH2-Ocus and NH2-OHcus are 0.48 and 0.31 eV, respectively, and the reaction energies are endothermic by 0.34 and 0.31 eV, respectively. Again, both of these two dehydro-

Figure 4. Energy diagrams for NHx dehydrogenation to (a) Obr and (b) Ocus.

genation pathways have late TSs. The reaction energy of NH2-OHcus is almost the same as the reaction barrier; the final state of this pathway is only 0.004 eV lower than the TS. Parts d and e of Figure 3 display the TS structures of NH2-Ocus and NH2-OHcus. We also modeled the NH2-cus dehydrogenation to an Obr atom on the oxygen-rich RuO2(110) surface. If an Ocus atom neighbors the adsorbed NH2-cus molecule, the adsorption of NH2-cus with the N-H bond pointing toward the Obr atom can be a stable minimum (Figure 2e).21 Therefore, in this initial state, the H atom of NH2-cus could migrate to the Obr atom directly; this process is different from pathway NH2-Obr. We denote this pathway as NH2-Obr-2. The reaction barrier and reaction energy of this step are 0.59 and 0.43 eV, respectively (Table 1); Figure 3f displays a top view of the TS. NH Dehydrogenation. The dehydrogenation of NHcus is relatively easier than those of NH3-cus and NH2-cus. In the four NHcus dehydrogenation pathways, only pathway NH-OHbr has a barrier (0.48 eV)sthe other three (NH-Ocus, NH-OHcus, and NH-Obr) are barrierless. Moreover, only the reaction energy of NH-OHbr is endothermic (by 0.47 eV)sit is only 0.01 eV lower than the TS; the others are exothermic, with reaction energies ranging from -0.78 to -0.93 eV (Table 1). In the three barrierless NHcus dehydrogenation pathways, both the initial and final states can be characterized in each pathway, but no maxima can be found on the potential energy surface during the H atom migration process. Figure 4 presents reaction energy diagrams for the NHx dehydrogenation to Obr and Ocus. Among the NHx dehydrogenation processes, the reaction pathway having the highest barrier is NH2-Obr. If thermal energy could overcome the barrier of NH2-Obr (0.86 eV), then all of the NHx dehydrogenation processes could occur on the RuO2(110) surface. This highest barrier of 0.86 eV on the RuO2(110) surface is still lower than those for NH3 dehydrogenation on Pt, Rh, and Pd surfaces, where the rate-determining barriers of the reactions (including

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TABLE 2: Geometric Parameters (Å) and Interaction Energies (eV) for the Oxidation Products species H2Ocus H2Obr NOcus N2-cus N2Ocus a

d[Ru-O(N)] 2.16 2.30 1.78 2.02 2.06

d(O-H) 0.97,1.07 0.98

d(N-O)

d(N-N)

Eint

1.12 1.14

-1.22 -0.70 -2.09 -0.53 -0.52

a

1.17 1.20

Bond length of hydrogen-bonded O-H unit.

bare surface and oxygen-assisted dehydrogenations) on these Pt-group metal surfaces range from 1.04 to 1.72 eV.8-12 In addition, a comparison of the dehydrogenation barriers reveals that most of the binding energies for NH3 on Pt-group metal surfaces are very low,8,9,11,12 which might favor NH3 desorption rather than dehydrogenation. In a previous study, we determined that the interaction energy of NH3-cus on the stoichiometric RuO2(110) surface (-1.56 eV)21 is much higher than the barrier for NHx dehydrogenation, suggesting that adsorbed NH3-cus species prefer dehydrogenation over desorption; the RuO2(110) surface can, therefore, provide a high turnover frequency during NH3 oxidation. According to our calculations, NHx dehydrogenation processes can occur on both stoichiometric and oxygen-rich RuO2(110) surfaces. Wang et al. found, however, in a study of the oxidation of NH3 on the RuO2(110) surface,14 that exposure of 2 L of NH3 to a stoichiometric RuO2(110) surface and heating to 470 K resulted in the NH3 molecules preferring desorption over oxidation. This phenomenon might be due to an insufficient number of Obr atoms being present for dehydrogenation at a high coverage of NH3-cus species; moreover, in our calculations, only pathway NH-OHbr was possible for H2Obr formation, although it is still a thermodynamically unfavorable process. In contrast, on the oxygen-rich RuO2(110) surface, Ocus atoms provide more capacity for dehydrogenation and more alternative H2O formation pathways via Ocus atoms, making further oxidation possible, as we discuss below. B. Oxidation Products. H2O Formation. Water is one of the products from the oxidation of NH3; both H2Ocus and H2Obr could form during the dehydrogenation pathways. The interaction energies of H2Ocus and H2Obr are -1.22 and -0.70 eV, respectively (Table 2). Parts a and b of Figure 5 present the structures of H2Ocus and H2Obr on the RuO2(110) surface. For H2Ocus, one of the H atoms is attracted by the neighboring Obr atom; this hydrogen bond results in a longer O-H bond length (1.07 Å) and a relatively high interaction energy (-1.22 eV). For H2Obr, two dative bonds exist between the water molecule and the surface. Greater coordination of H2Obr species to the surface does not, however, result in a higher adsorption energy. Indeed, the interaction energy of H2Obr is -0.70 eV, considerably lower than that of H2Ocus; the Ru-O bond length of H2Obr (2.30 Å) is also longer than that of H2Ocus (2.16 Å). In addition to the dehydrogenation of NHx species, H2Ocus can also be produced through the following OH disproportionation reactions:

OHcus-OHcus OHcus-OHbr

Figure 5. Adsorption geometries of NH3 oxidation products on the RuO2(110) surface: (a) H2Ocus, (b) H2Obr, (c) NOcus, (d) N2-cus, and (e) N2Ocus.

TABLE 3: Barriers (E‡, eV), Reaction Energies (∆E, eV), and Imaginary Frequencies (IMF, cm-1) for the Formation of H2O and Nitrogen Atom-Containing Oxidation Products pathway

E‡

∆E

IMF

OHcus-OHcus OHcus-OHbr Hdiff(cus-cus) Hdiff(cus-br) Hdiff(br-br) Ncus-Ocus Ncus-Obr Ncus-Ncus Ncus-NOcus

0.17 0.04 0.09 0.17 2.25 0.47 0.89 0.20 0.85

0.04 0.03 0.00 0.03 0.00 -1.82 0.03 -3.79 -0.98

i232 i527 i403 i872 i293 i610 i568 i579 i444

(OHcus-OHcus) and 0.04 eV (OHcus-OHbr), which are considerably lower than those for the NHx dehydrogenation pathways. Water formation through OH disproportionations during the oxidation of NH3 has also been proposed for an oxygenprecovered Pt(100) surface.10 We also calculated the energy requirement for H atom diffusion between surface oxygen species. For the diffusion from Ocus to Ocus [Hdiff(cus-cus)], the

OHcus + OHcus f H2Ocus + Ocus OHcus + OHbr f H2Ocus + Obr

The H2Ocus species in these two pathways are formed from neighboring OH species. Table 3 lists the energetics of these two pathways; parts a and b of Figure 6 present the structures of the TSs. Water formation through OH disproportionation occurs with relatively low reaction barriers, 0.17 eV

Figure 6. Top views of the TSs for the formation of NH3 oxidation products on the RuO2(110) surface: (a) OHcus-OHcus, (b) OHcus-OHbr, (c) Ncus-Ocus, (d) Ncus-Obr, (e) Ncus-Ncus, and (f) Ncus-NOcus.

Oxidation of Ammonia on RuO2(110) Surface energy barrier is 0.09 eV; from Ocus to Obr [Hdiff(cus-br)], it is 0.17 eV; for H atom diffusion from one Obr atom to another, however, the energy required to overcome the barrier is 2.25 eV [Hdiff(br-br)]. This result demonstrates that H atoms have high mobility via Ocus atoms on the RuO2(110) surface, and are localized on Obr atoms on the stoichiometric surface. The high barrier for H atom diffusion between Obr atoms might be another reason for the inhibited oxidation of NH3 on the stoichiometric RuO2(110) surface. The recombination of neighboring OHcus and OHbr species on the RuO2(110) surface, leading to the formation of H2Obr, is not feasible because there is no stable minimum for the structure H2Obr + Ocus. In other words, only H2Ocus is a possible product in OH disproportionation reactions. Furthermore, in the NHx dehydrogenation pathways, only NH-OHbr could possibly produce H2Obr, but it is a thermodynamically unfavorable reaction. As we mentioned above, the reaction barrier and reaction energy of NH-OHbr are 0.48 and 0.47 eV, respectively, meaning that the barrier for the reverse reaction is only 0.01 eV. Although the interaction energy of H2Obr is not high (-0.70 eV), the reverse reaction (decomposition of H2Obr) remains much more favorable than H2O desorption. From a comparison of all of the possible H2O formation pathways, we find that although the Obr atoms participated in the NHx dehydrogenation reactions, most of the H2O molecules desorbed from Rucus sites; therefore, the Ocus atoms were the major sources of oxygen for H2O formation. Formation of N-Containing Products. After all of the H atoms had been removed from the N atom of NH3, the formation of each of the following N-atom-containing products is possible: NOcus, N2-cus, and N2Ocus. Here, NOcus and N2-cus are the primary products; N2Ocus is the secondary product, because the production of N2O occurs after NOcus appears on the surface (formation mechanisms are discussed in next section). Figure 5c-e presents the structures of these N-atom-containing species adsorbed on the RuO2(110) surface; Table 2 lists their selected geometrical parameters. Among the oxidation products, the NOcus molecule is the most stable bonded species, having an interaction energy of -2.09 eV. The rate-determining step of the overall oxidation reaction should be the desorption of NOcus. Similar phenomena have also be found on platinum group metals (PGM): the high adsorption energy of NO on the surface makes the NO desorption to the gas phase the most energy-demanding step.29 The adsorbed NOcus species is bonded vertically on the Rucus site, with values of d(Ru-N) and d(N-O) of 1.78 and 1.17 Å, respectively. These values are consistent with those obtained from recent DFT calculations.30 Relative to the NO molecule in the gas phase, the value of d(N-O) of NOcus is slightly shortened by 0.02 Å. Another N-atom-containing product, N2-cus, is relatively weakly bonded to the surface; its interaction energy is -0.53 eV and the Ru-N bond length (2.02 Å) is significantly longer than that of adsorbed NOcus. The interaction between the surface and the N2-cus molecule causes the value of d(N-N) to be slightly longer (by 0.02 Å) than that of a free N2 molecule, due to electron transfer from the N2 species to the surface. Our calculated interaction energy and adsorption geometry for N2-cus are quite consistent with those reported by Kim et al. from low-energy electron diffraction (LEED) studies and DFT calculations.31 The characterization of N2O on PGMs has been studied for several years by Pe´rez-Ramı´rez and co-workers.29,32-35 N2O is one of the most important byproduct of ammonia oxidation over PGMs in TAP (temporal analysis of products) reactors.29,33-35 According to our calculations, N2O is another possible N-atom-containing product formed from the

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17415 oxidation of NH3 on the RuO2(110) surface. The N2Ocus molecule on the RuO2(110) surface is adsorbed vertically on the Rucus site via the N atom (Figure 5e). The interaction energy of N2Ocus is -0.52 eV; therefore, it is also a weakly bonded molecule, like N2-cus, on the surface. The following reaction pathways define the formation mechanisms of N-atom-containing products on the RuO2(110) surface:

Ncus-Ocus Ncus-Obr Ncus-Ncus Ncus-NOcus

Ncus Ncus Ncus Ncus

+ + + +

Ocus f NOcus Obr f NOcus Ncus f N2-cus NOcus f N2Ocus

Table 3 lists the reaction energetics and imaginary frequencies of each reaction TS; Figure 6c-f presents the structures of the TSs. The Ncus atom can be oxidized by surface Ocus (Ncus-Ocus) or Obr (Ncus-Obr) atoms to form a molecule of NOcus; the barriers of these two oxidation reactions are 0.47 and 0.89 eV, respectively; the reaction energies of Ncus-Ocus and Ncus-Obr are -1.82 and 0.03 eV, respectively. Compared with PGM surfaces, the formation barrier of NOcus on the RuO2(110) surface is higher than that on the Pt(100) surface (0.09 eV),29 but much lower than those on Pd(100) and Rh(100) surfaces (1.63 and 1.66 eV, respectively).29 If the two Ncus atoms are bound on adjacent Rucus sites, they can follow the Ncus-Ncus recombination process to form a molecule of N2-cus on the surface. This recombination has a low reaction barrier (0.20 eV) and a high exothermic energy (-3.79 eV). The reaction barrier for N atom recombination on the RuO2(110) surface is much lower than that on some other metal surfaces.12,36-38 Figure 7 presents the energy diagram of four Ncus reaction pathways, including the desorptions of N2-cus, NOcus, and N2Ocus. For the primary product formation pathways, the recombination of two Ncus atoms is favored both kinetically and thermodynamically; i.e., the lowest barrier and largest exothermic energy exist in the Ncus-Ncus recombination process. In addition, the high exothermic energy in the Ncus-Ncus process, combined with a low adsorption energy of N2-cus, results in high selectivity toward the production of N2. Of course, formation of NO molecules on the RuO2(110) surface is also possible. Of the two NOcusformation pathways, Ncus-Ocus is favored over Ncus-Obr. Figure 7 reveals that the relative energy of the TS in Ncus-Ocus (0.47 eV) is higher than that of NO(g) (0.27 eV), implying that the NOcus species formed via pathway Ncus-Ocus prefers to desorb. In contrast, the Ncus-Obr pathway is kinetically and thermodynamically unfavorable. Therefore, the Ocus atoms are the major oxygen source for NOcus formation, similar to our conclusion for the production of H2O. During the formation and desorption of N2-cus species on the RuO2(110) surface, the required energies (0.20 eV for formation, 0.53 eV for desorption) are both lower than the highest reaction barrier of all the NHx dehydrogenation processes (0.86 eV), suggesting that N2 molecules will be produced simultaneously after dehydrogenation. In a thermal desorption spectroscopy (TDS) analysis of the oxidation of NH3 on the RuO2(110) surface, in addition to a signal for the desorption of N2 at ca. 200 K, the major peak for the desorption of N2 in TDS is located at a much higher temperature (ca. 420 K).14 Therefore, there must be another phenomenon dominating the desorption of N2 from the RuO2(110) surface. Indeed, we found that the N2 desorption peak observed at high temperature was controlled by the diffusion of Ncus atoms on the surface. The barrier for diffusion of a Ncus atom from one Rucus site to another is 1.44

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Figure 7. Energy diagram for the formation and desorption of N-atom-containing products.

eV, an energy barrier that is higher than the desorption energy of H2Ocus (1.22 eV); therefore, the desorption temperature for N2 molecules should be higher than that for H2O molecules during TDS.14 If two NH3 molecules adsorb initially on neighboring Rucus sites, the N2 molecule will be produced immediately after dehydrogenation, and then desorb immediately; if the Ncus atom is isolated on the surface, the energy required for Ncus diffusion will result in the higher desorption temperature for N2-cus. The secondary N-atom-containing product on the RuO2(110) surface, N2Ocus, is produced via the recombination of Ncus and NOcus. The reaction barrier of this pathway is 0.85 eV, which is higher than that on Pt(100) (0.68 eV)29 and much lower than those on Pd(100) and Rh(100) (1.51 and 1.79 eV, respectively),29 and the reaction energy is exothermic by -0.98 eV. After N2Ocus is formed on the RuO2(110) surface, it could desorb from the surface, or decompose into N2-cus and Ocus. For the N2Ocus decomposition on the RuO2(110) surface, the vertically bonded N2Ocus on the surface is highly symmetric; the N-O bond dissociation and the oxygen diffusion will break the symmetry of N2Ocus. Moreover, for the N-bonded N2Ocus, the O atom is 4.40 Å from the surface; this long distance between the O and Rucus atoms will also make the N2O decomposition difficult. The calculated reaction barrier of N2O decomposition is 1.35 eV; that is, N2Ocus will desorb right after its formation.39 In conclusion, our calculations regarding N2O formation on the RuO2(110) surface reveal that the energetically favored process is desorption rather than decomposition. From a kinetics point of view, however, N2-cus is the most favored pathway toward N-atom-containing products; it will rapidly reduce the possibility of obtaining the initial state of N2Ocus formation (Ncus + NOcus). Moreover, the calculations reveal a lower barrier exists for the Ncus atom to form N2-cus (0.20 eV) or NOcus (0.47 eV) rather than N2Ocus (0.85 eV). This finding explains why, in the ultrahigh vacuum (UHV) experiment reported by Wang et al.,14 no N2O could be detected from the NH3 oxidation on the RuO2(110) surface. During the oxidation of NH3 on the RuO2(110) surface, Ocus atoms play an important role. In the dehydrogenation pathways, the reaction cannot complete without the presence of Ocus atoms on the surface; during the formation of H2O and NO, Ocus atoms are the most favored oxidants for the oxidation products. Wang et al. found experimentally14 that the yield of NO molecules was dominated by the coverage of Ocus atoms; when more Ocus

atoms were present on the surface, more NO molecules were produced. According to our calculations, even though the formation barrier for NOcus is not very high, some of the surface Ocus atoms become OHcus or H2Ocus species after dehydrogenation of the NHx species. Thus, in addition to competition with N2-cus formation, the production of NOcus is also inhibited by the presence of surface OHcus and H2Ocus species. Obviously, one way to increase the selectivity of NOcus production is to increase the number of Ocus atoms on the RuO2(110) surface, which not only provides more O atoms for Ncus + Ocus recombination but also inhibits the diffusion of Ncus atoms and reduces the probability of two Ncus atoms on adjacent Rucus sites producing a molecule of N2. Our calculated results are in good agreement with the experimental observations and kinetic models reported previously by Wang et al.14 4. Conclusion We have employed DFT calculations to characterize the possible pathways for the reactions of NH3 on RuO2(110) surfaces, including dehydrogenation and oxidation. From calculations of the energetics of several possible dehydrogenation and oxidation pathways, we conclude that the oxidation of NH3 can occur on both stoichiometric and oxygen-rich RuO2(110) surfaces. On the stoichiometric surface, however, the number of Obr atoms is insufficient for successful dehydrogenation, especially at a high coverage of NH3-cus; moreover, there is no effective H2Obr formation pathway when Ocus atoms are absent from the surface. Therefore, NH3 molecules prefer desorption over oxidation on the stoichiometric RuO2(110) surface. Not only do the Ocus atoms make the dehydrogenation of NH3 possible on the RuO2(110) surface, they are also the major oxidant in the oxidation reaction; i.e., the oxygen atoms in the produced NO and H2O species arise from Ocus atoms. For the oxidation products, the formation of gaseous H2O and NO molecules from NH3-cus are both determined by their desorption energies from the surface; the desorption energies are higher than any of the reaction barriers; the formation of N2 molecules, which have a very low desorption energy, is, however, controlled by the diffusion of Ncus atoms. The high diffusion barrier of Ncus atoms results in a high desorption temperature for N2-cus, as has been observed in a TDS experiment.14 In addition, our calculations indicate selectivity toward the N-atom-containing products N2-cus, NOcus, and N2Ocus. The selectivity toward

Oxidation of Ammonia on RuO2(110) Surface the primary N-atom-containing products (N2-cus, NOcus) could be obtained by controlling the coverage of Ocus atoms on the RuO2(110) surface; a high selectivity toward NOcus could be achieved easily by increasing the Ocus coverage, which blocks the diffusion of N atoms to form N2 molecules. We also characterized the mechanism of formation of N2Ocus on the RuO2(110) surface; our calculations revealed that the formation of this secondary N-atom-containing product is kinetically unfavorable. The suggested reaction mechanisms that we obtained through our DFT calculations provide reasonable explanations for the experimentally observed selectivity of the products formed on RuO2(100) surfaces.14 Acknowledgment. We thank the National Science Council of Taiwan (NSC 97-2113-M-011-001 and NSC 94-2120-M-011001) 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: Tables S1 and S2 list the benchmark of the calculated interaction energy of NH3-cus. Figure S1 illustrates a side view of the surface model used in this study and the definition of the number of layers. Tables S3-S6 list selected geometrical parameters of the initial, transition, and final states of the reactions leading to the formation of NHx dehydrogenation and oxidation products. Figures S2-S5 illustrate the optimized structures of the initial and final states involved in the examined reactions. Figure S6 displays the TS geometry and the energy diagram for N2Ocus decomposition on the RuO2(110) surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gang, L.; Anderson, B. G.; van Grondelle, J.; van Santen, R. A.; van Gennip, W. J. H.; Niemantsverdriet, J. W.; Kooyman, P. J.; Koester, A.; Brongersma, H. H. J. Catal. 2002, 206, 60. (2) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal., B 1998, 18, 1. (3) Yin, S. F.; Xu, B. Q.; Zhou, X. P.; Au, C. T. Appl. Catal., A 2004, 277, 1. (4) Stolbov, S.; Rahman, T. S. J. Chem. Phys. 2005, 123, 204716-1. (5) Christensen, C. H.; Johannessen, T.; Sørensen, R. Z.; Nørskov, J. K. Catal. Today 2006, 111, 140. (6) Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; WileyVCH: Weinheim, Germany, 2005. (7) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991.

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