Combined Density Functional Theory and Kinetic Monte Carlo Study

Article pubs.acs.org/JPCC

Combined Density Functional Theory and Kinetic Monte Carlo Study of Selective Oxidation of NH3 on Rutile RuO2(110) at Ambient Pressures Syed Islamuddin Shah, Sampyo Hong, and Talat S. Rahman* Department of Physics, University of Central Florida, Orlando, Florida 32816, United States ABSTRACT: We have carried out combined density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations for ammonia oxidation on RuO2(110) using a database of 24 reaction processes (36 processes if reverse processes are counted separately) and compared the selectivity for reaction products under ultrahigh vacuum (UHV) and ambient pressures at selected temperatures. We find that in keeping with earlier experimental and theoretical findings NO selectivity is almost 100% above 600 K in UHV. However, this selectivity disappears for reactions at ambient pressures. We relate the lack of selectivity under ambient pressures to the propensity for NO to convert to N2O and to active recombination of Ncus + Ncus owing to the abundant supply of N species in ambient pressure as a result of active NHx decomposition by plenty of O species on the RuO2(110) surface. study of NH3 oxidation on RuO2(110),5 mimicking UHV conditions, we showed that NO selectivity was almost perfect, in agreement with the experiment of Wang et al.3 In the present work, we have thus revisited our DFT + KMC simulation of ammonia oxidation reactions on RuO2(110). We have calculated using DFT the rates for a set of 36 reactions to include the intermediates found in experiments performed under ambient pressures. We have then carried out KMC simulations to examine the temperature and pressure dependencies of product selectivity, using not only our database of activation energies but also those that are available in the literature, to provide an overall comparison of their implications. Our previous KMC simulations of NH3 oxidation on RuO2(110) were limited to reactions on the 1-D chain of Rucus sites with 13 processes (18 containing reverse processes). As will be seen, while the model and the results presented here involve many more intermediate reactions and the role of the Obr sites, our conclusions for selectivity under UHV conditions remain qualitatively unchanged. At the same time we provide insights into how the selectivity becomes questionable when reactions are performed at ambient pressures. Since our earlier KMC simulations of NH3 oxidation on RuO2(110), five publications have appeared on the topic,4,6−9 some of which use DFT to calculate the adsorption geometry and reaction energetics for either NH3 or NO on the RuO2(110) surface. Very briefly, the conclusions from these later publications are: (1) NO desorption is the rate-limiting step, (2) the contribution of bridge sites in the reaction is

I. INTRODUCTION Catalytic ammonia oxidation via the Ostwald process is an industrially important reaction for producing nitric acid, which is an important inorganic intermediate for the production of fertilizers and other chemical products such as explosives.1 Platinum−rhodium (with 5−10% Rh content) serves as a catalyst for this oxidation reaction in temperature range of 1070−1220 K and at high pressure. However, the high cost of Pt−Rh together with the high operating temperatures and frequent replacement of the catalyst (every 3−6 months depending on operating pressure) call for their substitution by more cost-effective materials. In this regard the recent finding of RuO2 as an excellent oxidation catalyst has been welcome news (see review2). Furthermore, RuO2(110) was shown to be an excellent catalyst for ammonia oxidation, in experiments undertaken in ultrahigh vacuum (UHV) such that its NO selectivity was found to be as high as that of Pt−Rh catalysts at a much lower temperature of 530 K.3 If this selectivity would be maintained under industrial catalytic conditions, RuO2 could be a possible substitute for the expensive platinum-based catalysts used in industry. However, very recent experiments by Péreź et al.4 on polycrystalline rutile RuO2 samples carried Ramirez out at ambient pressures find that N2 rather than NO is the dominant product with selectivity of about 80%, suggesting that there is a pressure gap and also possibly a material gapactive RuO2 phase or facet other than (110)in ammonia oxidation on RuO2. These sets of experiments provide perfect motivation for simulations of the system reaction kinetics under ambient conditions, using as accurate a technique as feasible, to obtain a rationale for the dependence of reaction selectivity on pressure. This is particularly essential since in a previous density functional theory and kinetic Monte Carlo (DFT + KMC) © 2014 American Chemical Society

Received: August 6, 2013 Revised: January 17, 2014 Published: February 10, 2014 5226

dx.doi.org/10.1021/jp407865e | J. Phys. Chem. C 2014, 118, 5226−5238

The Journal of Physical Chemistry C

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negligible, and (3) the lack of N2O formation in UHV results from slow diffusion of N species.4,6−9 Furthermore, N2 dominance in experiments performed at ambient pressure is attributed to the facile decomposition of N2O species to N2.4 Most of these conclusions, like many others in the literature, are based only on comparisons of the activation energies for various processes. They are thus restricted to considerations of thermodynamics at zero temperature and zero pressure and do not include effects of reaction kinetics. As we know, reactivity and selectivity of a catalytic process cannot be judged only in terms of energy barriers of a few key reactions. Rather, consideration of competition among various processes, both key and intermediate, is essential to obtain the full picture. Toward this end KMC simulations have been employed, particularly to reactions on RuO2(110). Since this surface exhibits a confinement of reaction sites along the onedimensional rows of undercoordinated Ru sites (Figure 1),

Figure 2. Reaction steps in NH3 oxidation on RuO2(110).

B. DFT Calculations. We have performed scalar-relativistic DFT calculations in the ultrasoft (US) pseudopotential (PP)14 and plane wave basis set methods to calculate the structure and energetics for ammonia oxidation reactions on RuO2(110). Our USPP for Ru includes 4d, 5s, and 5p states as the valence states with a scalar relativistic correction, while those for H, N, and O include 1s (in case of H) or 2s and 2p states (in the case of N and O) as the valence states.5 We used the Perdew−Burke− Ernzerhof (PBE) functional for exchange−correlation energy15 and a (3 × 6 × 1) grid for k-point sampling in the Brillouin zone. All calculations were performed using the Quantum Espresso (QE)16 code. Our slab consists of three O−RuO−O trilayers, separated by 18 Å of vacuum. NH3 and other molecules of interest were adsorbed only on one side of the slab. All atoms in the supercell underwent ionic relaxation to their equilibrium configurations. To simulate the reaction process of type A + B = C, we used a (3 × 1) surface unit cell of dimension 9.60 × 6.56 Å2, which includes three Rucus sites. Further description of the calculation setup can be found elsewhere.5 We have calculated the adsorption energy for species X using Ead(X) = E(Xsurf/RuO2) − E(RuO2) − E(Xgas), where E(Xsurf/ RuO2) is the spin-polarized total energy of X adsorbed on RuO2(110), and both E(RuO2) and E(Xgas) are spin-polarized total energy of clean RuO2(110) and X in the gas phase, respectively. Zero-point energy (ZPE) corrected adsorption energy, EadZPE(X/RuO2), is then obtained by adding ZPE(Xad) − ZPE(Xgas) to the adsorption energy, where ZPE(X) is given by the sum of all (nonimaginary) vibrational frequencies of species X divided by 2. C. KMC Algorithm. We evaluate the rate of the reaction of interest on the basis of kinetic expressions derived from transition state theory (TST).17 The transition state (TS) is by definition the saddle point in the minimum (free) energy path (MEP), which consists of initial state (IS), saddle point, and final state (FS) as shown in Figure 3. The TST reaction rate, rTST, is the rate of forward crossing of the TS and has the form: eb rTST = Ae−ΔF /kT, where A and ΔFeb are, respectively, the prefactor and the energy barrier (a.k.a. activation energy) (cf. Figure 3). We evaluate the adsorption rate of ammonia and oxygen using rads= sP/σ(2πmkT)1/2, where s, P, σ, m, k, and T are the sticking coefficient, partial pressure of ammonia or oxygen, site density, mass of ammonia or oxygen, Boltzmann constant, and temperature, respectively. For purposes here, s = 1 and site density σ is calculated from the surface unit cell in question. It is the partial pressure P that distinguishes the UHV and ambient cases in our calculations. For UHV conditions P(NH3) is set to 10−7 mbar, while P(O2) varies from 0.5 × 10−7 to 20 × 10−7 mbar. To mimic the ambient conditions in the

Figure 1. Stick-and-ball model of the stoichiometric rutile RuO2(110) surface.

KMC simulations are particularly suited for description of such localized reactions.10,11 However, as pointed out by Over et al.12,13 it is important to note that CObr also plays a significant role in CO oxidation on RuO2(110). In many aspects NH3 oxidation on RuO2(110) is similar to CO oxidation as main reactions occur mostly along the linear chain of Rucus but with a significant contribution from Obr. In the next section we present the details of our modeling and simulations, which include a list of our calculated energy barriers and prefactors for the reactions of interest. The results and discussions are presented in Section III, while the conclusions are summarized in Section IV.

II. THEORETICAL METHODS A. Model System. We present a stick-and-ball model for the stoichiometric rutile RuO2(110) surface in Figure 1. The RuO2(110) surface exposes rows of undercoordinated Ru (Rucus) and O atoms (Obr), both of which have unpaired bonds along the surface normal. Shown in Figure 1 are also fully coordinated Ru (Ru br ) and O atoms (O layer ), whose coordination is 6 and 3, respectively. As has already been discussed in several earlier publications,4,6−9 NHx and O2 adsorb on Rucus (dissociatively in the case of O2). Initially, decomposition (or dehydrogenation) of NHx occurs on RuO2(110) via reaction with surface O species: Ocus adsorbed on top of Rucus and Obr adsorbed on top of Rubr. Reaction of NHx with the O species produces NO, N2O, OH, and H2O. Some of the elementary reaction steps are shown in Figure 2. 5227



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dependence is implemented as a factor eαθ/kT in the rate expression, where α is the adsorption energy change (in eV) from low to high coverage and θ is the coverage of the adsorbate on RuO2(110) (0 ≤ θ ≤ 1). From the calculated adsorption energy for 1/3 and 1 monolayer (ML) coverage, we find α to be 0.34 eV for NH3cus and 0.16 eV for NOcus. D. Calculations of Activation Barriers and Prefactors. We have employed the climbing image nudged elastic band (CI-NEB) method18 to calculate a minimum energy path (MEP), using eight images for each reaction. During CI-NEB calculations we fix the atoms in the two bottom trilayers at their bulk positions. Thus, we relax atoms only in the topmost trilayer and the molecules adsorbed on it. To obtain the spincorrected total energy for transition-state geometry, we perform a single spin-polarized self-consistent calculation for TS geometry obtained from nonspin-polarized calculations. Total energies of the IS, FS, and TS are corrected by the zero-point energy. Energy barrier (ΔFeb) for the forward reaction is FTS − FIS and that for the reverse reaction FTS − FFS. The total energy difference ΔF (between initial and final states) is simply FFS − FIS. For a spontaneous process, we set the activation energy for the forward process to zero and that for the reverse process to the total energy difference ΔF. We use a simplified formula (Vineyard) for evaluation of prefactors:19 A = Πi3NωiIS/ ωTS Π3N−1 i i , where ωi are the harmonic vibrational frequencies. We use the standard prefactor (1013 s−1) for some processes, for example, desorption and spontaneous processes, for which reliable vibrational frequencies are not available from DFT calculations. E. Databases of Reaction Processes. In this study, we have carried out KMC simulations using three databases: one from our present calculations, one from Wang et al.,7 and ́ et al.4 Our present database for another from Pérez-Ramirez

Figure 3. Potential energy surface and KMC energy barriers.

́ et al.4 (O2/NH3 feed ratio = 2, 5, experiments of Pérez-Ramirez 4 10), we set P(NH3) to 20 mbar and P(O2) to 40, 100, and 200 mbar in the temperature range 573−773 K. Note that in the ́ et al.4 NH3 pressure was varied experiments by Pérez-Ramirez from 20 to 50 mbar and that of O2 from 100 to 200 mbar in the temperature range 573−773 K. A flowchart of our KMC algorithm can be found elsewhere.5 Our KMC mesh is 30 × 40, i.e., 40 alternating columns of Rucus and Obr chains, each chain being made of 30 equivalent sites. We collect statistics after 108 KMC steps and continue until ∼1012 KMC steps. Our reaction rates and selectivities are extracted from steady-state statistics. As in our previous study,5 we take into consideration coverage dependence of the adsorption energy of NH3 and NO in our KMC simulations. In our code, this coverage Table 1. Our Calculated Database of 24 Processesa

a

#

reaction process for (110) surface

ΔE (eV)

Ea (eV)

νi (1013 s−1)

A (1013 s−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

NH3(gas) → NH3cus O2 → Ocus + Ocus NH3cus + Ocus → NH2cus + OHcus NH2cus + OHcus → NHcus +H2Ocus NH2cus + Ocus → Ncus + H2Ocus NHcus + Obr → Ncus + OHbr NHcus + Ocus → Ncus + OHcus NHcus + OHcus → Ncus + H2Ocus Ncus + Ocus → NOcus NOcus → NO(gas) Ncus → Ncus (diffusion) NOcus → NOcus (diffusion) Ocus → Ocus (diffusion) OHcus → OHcus (diffusion) Ocus → Obr (diffusion to O vacancy) Ncus + Ncus → N2cus N2cus → N2(gas) H2Obr → H2O(gas) H2Ocus → H2O(gas) OHbr + Obr → Obr + OHbr OHbr + OHbr → H2Obr + Obr NOcus + Ncus → N2Ocus N2Ocus → N2cus + Ocus N2Ocus → N2O(gas)

−1.46 −1.26 0.34 0.48 −0.41 −2.02 −0.82 −0.65 −2.89 1.72 0.0 0.0 0.0 0.0 −1.37 −5.19 0.58 0.85 1.30 −0.02 1.01 −1.34 −0.21 0.47

0.0 0.0 0.55 0.71 0.37 0 0 0 0.18 1.72 1.13 1.42 1.05 0.91 0.66 0.27 0.58 0.85 1.30 2.23 2.21 0.79 0.81 0.47

0.74 0.88 0.31 0.20 0.22 0.43 1.08 0.73 1.63 -

1.0 1.0 0.23 1.0 1.0 1.0 1.0 1.0 0.28 1.0 0.35 0.22 0.31 0.19 1.0 0.4 1.0 1.0 1.0 1.0 1.0 0.087 0.087 1.0

Total energy change (ΔE), energy barrier (Ea), imaginary frequency (νi), and prefactor (A) are shown. 5228



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Table 2. Wang et al.’s Database Adapted from Reference

a

7a

#


ΔE (eV)

Ea (eV)

νi (1013 s−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

NH3(gas) → NH3cus O2 → Ocus + Ocus NH3cus + Ocus → NH2cus + OHcus NH2cus + Ocus → NHcus + OHcus NH2cus + Obr → NHcus + OHbr NH2cus + OHcus → NHcus + H2Ocus NHcus + Ocus → Ncus + OHcus NHcus + Obr → Ncus + OHbr NHcus + OHcus → Ncus + H2O NHcus + OHbr → Ncus + H2Obr Ncus + Ocus → NcusO Ncus + Obr → NOcus Ncus + Ncus → N2cus NcusO → NO(gas) NcusO + Ncus → NcusNO H2Ocus → H2O(gas) H2Obr → H2O(gas) N2cus → N2(gas) NcusNO → N2O(gas) NcusNO → Ocus + N2cus OHbr + Obr → Obr + OHbr

−1.56 −1.26 0.62 0.34 0.11 0.31 −0.82 −0.78 −0.93 0.47 −1.82 0.03 −3.79 2.09 −0.98 1.22 0.70 0.53 0.52 −1.11 0.0

0.0 0.0 0.71 0.48 0.86 0.31 0.00 0.00 0.00 0.48 0.47 0.89 0.20 2.09 0.85 1.22 0.70 0.53 0.52 1.35 2.25

0.47 2.69 0.76 2.04 1.83 1.70 1.74 1.33 -

The symbols are defined as in Table 1.

́ et al.’s Database Adapted from Reference 4a Table 3. Pérez-Ramirez

a

#


ΔE (eV)

Ea (eV)

νi (1013 s−1)


ΔE (eV)

Ea (eV)

νi (1013 s−1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

N(g) → Ncus O(g) → Ocus Ncus + Ncus → N2(gas) Ncus + Ocus → NOcus Ncus + Ocus → ONcus NOcus → NO(gas) ONcus→ NO(gas) Ocus + Ocus → O2cus O2cus → O2(gas) NOcus + Ncus → N2Ocus N2Ocus → N2O(gas) NOcus + Ncus → NNOcus NNOcus→ Ocus +N2(gas) NOcus → ONcus

−3.41 −1.87 −0.39 1.78 0.31 0.50 0.67 −1.16 0.26 −0.85 −1.42 1.45

0.0 0.0 1.11 1.01 1.01 1.78 0.31 1.11 0.67 1.30 0.26 1.04 0.31 1.63

1.94 1.83 1.84 1.75 1.17 0.74 1.60 1.51

N(g) → Ncus O(g) → Ocus Ncus + Ncus → N2(gas) Ncus + Ocus → NOcus Ncus + Ocus → ONcus NOcus → NO(gas) ONcus → NO(gas) Ocus + Ocus → O2cus O2cus → O2(gas) NOcus + Ncus → N2Ocus N2Ocus → N2O(gas) NOcus + Ncus → NNOcus NNOcus → Ocus +N2(gas) NOcus → ONcus

−4.14 −2.18 −0.82 1.49 0.11 0.67 0.23 −1.42 0.11 −0.68 −1.31 1.37

0.0 0.0 0.58 0.72 0.72 1.49 0.11 0.96 0.23 1.23 0.11 0.76 0.21 1.66

1.38 1.60 1.60 1.61 0.40 1.27 1.38 0.51

The symbols are defined as in Table 1.

For the reader to appreciate the differences in the energy barriers present in the three databases of interest here, we have summarized those from Wang et al.7 in Table 2 as abstracted from the original publication.7 It consists of 21 processes (35 processes if reverse processes are counted separately). We have neglected from the original database six processes for which both forward and reverse barriers were small (∼0.1 eV or less) or the reverse barrier was much smaller than the forward barrier. For example, NH3cus + Obr has a small reverse barrier of 0.01 eV and forward barrier of 0.44 eV. We did test KMC runs for these reactions and confirmed that the forward process is definitely favored. Moreover, since the reverse barrier is one of the lowest barriers in the database, it gets picked right after the forward process, thus creating a long loop of forward and reverse steps of these processes. Similarly, OHcus + OHcus and OHcus + OHbr are not included as their reverse and forward barriers are not only small but also differ only by 0.03 eV. Including such processes in KMC simulations creates an

NHx decomposition on RuO2(110) consists of 24 processes (36 processes if reverse processes are separately counted), the list of which is presented in Table 1 together with the associated energies and prefactors. The first 13 reactions (except rows 5, 6, and 15) are the ones that were considered in our previous work.5 Note that there are small differences in some of the energy barriers for the first 13 processes from those in our previous work,5 as the method for calculating them was “drag” rather than CI-NEB that is employed in the present work. Similarly in the current study H2O desorption is treated as a nonspontaneous process, while it was assumed to be spontaneous in our previous work.5 Another point of deviation from our previous work concerns the reaction of NH2cus with Ocus (row 5 in Table 1) which we find to be a one-step process leading to formation of Ncus: NH2cus + Ocus → Ncus + H2Ocus. We had previously found it to be a two-step process, with the initial step: NH2cus + Ocus → NHcus + OHcus. 5229



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studies report larger energy barriers (0.717 − 0.75 eV6) and larger total energy changes from IS to FS (0.627 and 0.76 eV6). Even though the calculated activation energy for the first step (0.55 eV) is small as compared to most other activation energies in Table 1, its even smaller reverse barrier (0.21 eV) makes the first step of N−H scission a slow process. Its occurrence is about 2−4 events/site/s at 723 K at ambient pressure. In the next stage, NH2 can be dehydrogenated either in two sequential steps or in a single concerted step. The final product is N regardless of the route it takes. In the sequential route (Figure 5), N−H scission is initiated by the formation of a H···

endless cycle of forward and reverse steps, thus making KMC simulations intractable. In the literature, there are discussions about workarounds for overcoming such low barrier problems, but it still remains an unresolved and challenging issue. ́ et al.4 is presented in Similarly, the database of Pérez-Ramirez Table 3 and consists of 12 processes (processes 1 and 2 are added by us to their original database), each for RuO2(110) ́ et al.4 and RuO2(101). Note that the database of Pérez-Ramirez includes only N and O recombination and their secondary reaction steps. Thus, it does not have NHx dehydrogenation steps. Therefore, KMC simulations using this database cannot be directly compared with the other databases, and its results need ́ et al.4 and to be interpreted with care. Since Pérez-Ramirez 7 Wang et al. do not report prefactors for their reactions, we have used the standard prefactor (1013 s−1) for all processes in their databases. As described earlier, we have calculated prefactors for most reaction processes considered by us, as long as it was feasible to obtain reliable vibrational frequencies for the TS.

III. RESULTS AND DISCUSSION We discuss in this section first the geometric structure of the reactants and the energetics of the reaction steps of NHx decomposition on RuO2(110) and compare them with results from the two other studies that are summarized in Tables 2 and 3. In addition to hydrogen abstraction processes, we consider the processes of formation, dissociation, desorption, and diffusion of the species of interest, pointing to their geometrical structure in the initial, transition, and final states. Discussion of KMC results and their implications continues at the end of this section. In all figures below for the reaction geometries the Ru atoms are colored light green and the substrate oxygen atoms red, while the adsorbed oxygen atom is depicted in brown. The nitrogen atom is presented in yellow, whereas hydrogen atoms are blue. A. Structure and Energetics of NHx Oxidation Steps on RuO2(110). (1). Ocus-Induced H Abstraction. The entire process of NH3 decomposition on RuO2(110) can be summarized in a single step: 4NH3 + 3O2 → 4N + 6H2O. The reaction is initiated by hydrogen bonding (H···O). As a result, three N−H bonds are broken, and three O−H bonds are formed. The first N−H scission of NH3cus on RuO2(110) is initiated by Ocus through H···Ocus interaction, whose bond length is 1.99 Å, and as a result NH2cus is produced as an intermediate. The IS, TS, and FS for this step are presented in Figure 4. The bond length of H−O is 1.59 Å for TS and 0.98 Å for FS. This is an endothermic reaction with energy deficiency of 0.34 eV. The energy barrier of this step is 0.55 eV. Other

Figure 5. Reactant geometry for the NH2cus + OHcus reaction. (a and b) Top and side view for IS. (c and d) Top and side view for TS. (e and f) Top and side view for FS.

Ocus bond (2.04 Å). As soon as one H is transferred to Ocus, the other H starts to rotate to form a H···Obr bond (2.64 Å). However, TS is reached just before this hydrogen bond forms (Figures 5c and 5d). In FS, this remaining H finishes rotation of 118° to form H···Ocus (2.14 Å) (see Figures 5e and 5f). As a result, NHcus and H2Ocus are produced. Note that the water molecule also forms a hydrogen bond with Obr (H···Obr). This is an endothermic reaction (ΔE = 0.48 eV), and the energy barrier is 0.71 eV. The other studies report barriers ∼0.3 eV.6,7 The last remaining N−H bond is broken by either OHcus or Ocus. Regardless of the reaction counterpart, they are both spontaneous and downhill reactions with large energy change.6,7 In sum, the whole sequential step can be summarized as NH2cus + OHcus + Ocus (or OHcus) → NHcus + H2Ocus + Ocus (or OHcus) → Ncus + H2Ocus + HOcus (or H2Ocus). In the concerted dehydrogenation route, which can be summarized as NH2cus + Ocus → Ncus + H2Ocus, two N−H scission events occur via a single Ocus in a concerted motion, as in Figure 6. In fact, initially there are two H···O bonds: H···Obr with a bond length of 2.77 Å and H···Ocus with a much shorter bond length of 2.08 Å (Figures 6a and 6b). Reaction occurs first along the shorter hydrogen bond. In the concerted motion, the moment the first H is transferred to Ocus, forming OHcus, the second H turns by 25° forming a H···Obr bond (Figures 6c and

Figure 4. Reactant geometry for the NH3cus + Ocus → NH2cus + OHcus reaction. (a and b) Top and side view for IS. (c and d) Top and side view for TS. (e and f) Top and side view for FS.

Figure 6. Reactant geometry for NH2cus + Ocus reaction. (a and b) Top and side view for IS. (c and d) Top and side view for TS. (e and f) Top and side view for FS. 5230



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simple dragging method.) Other studies report a much higher energy barrier in the range of 0.477 and 1.01 eV4 (Table 4). Thus, the energy barrier of NO formation is controversial. Similarly, our calculated desorption barrier for NO is 1.72 eV (1.83 eV without zero-point energy correction), and other studies report NO desorption barriers of 1.78,4 1.96,6 2.09,7 and 2.25 eV.8 However, the experimental NO desorption barrier on RuO2(110) estimated for low coverage from TDS spectra3 using the Redhead equation20 is 1.24 eV if the desorption prefactor is chosen to be 1013, 1.52 eV if it is 1016, 1.61 eV if it is 1017, and 1.69 eV if 1018 is used. The desorption barrier of 1.97 eV is obtained only if the desorption prefactor is as high as 1021. Considering that the experimental desorption prefactor of NO on Pt(110) and Pt(111) is 1016 s−1,21 a reasonable guess for the prefactor for NO desorption on RuO2(110) would be in the range of 1013 and 1016 s−1 which would place the desorption barrier in the range of 1.24−1.52 eV. While all theoretical NO desorption barriers (1.72−2.25 eV) are higher than the experimental value, our calculated value (1.72 eV) is the closest to the experimental one. Note that NO desorption is the rate-determining step in ammonia oxidation on RuO2(110).3,4,6−9 The energy barrier for N 2 formation via N + N recombination is 0.27 eV. The desorption of thus-formed N2 is activated with an energy of 0.58 eV. As seen from Table 4, there is a large discrepancy among theories regarding N2 formation barriers as in the case of that for NO formation, although the former’s transition geometry is simple. The IS, TS, and FS are as shown in Figure 8. (4). N2O Formation. Once NOcus is formed, it can either desorb as NO or recombine with Ncus to form N2Ocus. (Another possibility is to recombine with Ocus to form NO2. However, NO2 is never observed in experiments.3,4) The TS and FS for N2O formation are presented in Figure 9. Recombination of NOcus with Ncus is an exothermic reaction with energy barrier of 0.79 eV (0.76 eV with zero-point correction). In this reaction, horizontally inclined (parallel to the surface) N2O species (Figure 9b) is formed as an intermediate. While this horizontally inclined N2O has lower energy than IS (not shown) by 1.08 eV, its energy is higher by 0.26 eV than that of vertically aligned N2O (Figure 9c), which is the FS. Our calculated energy barrier for the whole reaction is 0.79 eV and is in close agreement with that of Wang et al.7 (0.85 eV), ́ et al.4 report much whereas Schneider et al.8 and Pérez-Ramirez larger energy barriers (1.22 and 1.30 eV, respectively). The energy difference between IS and FS is also notable. Ours is −1.34 eV, but other studies report a smaller value in the range of −0.568 and −1.16 eV.4 In terms of the geometry of N2O, our calculated structural parameters are in good agreement with those of Wang et al.7 (see Table 4). Interestingly, Péreź et al.4 report a bidentate N2O (denoted as NNOcus in Ramirez Table 3) in addition to the monodentate, vertical N2O species. Their bidentate N2O, which is similar to our horizontal N2O in Figure 9b, binds to two neighboring Rucus atoms via both N and O ends. (5). N2O Decomposition and Desorption. N 2O on RuO2(110) can either directly desorb as N2O or decompose to N2. For the direct desorption, N2O needs to overcome an energy barrier of 0.47 eV, which is in good agreement with that ́ et al.4 report of Wang et al.7 (0.52 eV), whereas Pérez-Ramirez a much smaller desorption barrier (0.26 eV). In the decomposition route to N2 (Figure 10), it needs to overcome an energy barrier of 0.81 eV. We find this step to be exothermic

6d). In the TS, the hydrogen bond length is 2.12 Å. In FS, the H···Obr is replaced by H−Ocus forming water as a final product. This entire concerted step is an exothermic reaction (ΔE = 0.41 eV) with energy barrier of 0.37 eV. According to our KMC simulations, since the sequential route faces a larger forward barrier with smaller reverse barrier, its contribution to N−H scission is effectively zero. However, the concerted route is kinetically favored and thus becomes the major route for NH2cus decomposition with the rate of about 4 events/site/s at ambient pressures. (2). Obr-Induced H Abstraction. In principle, NH3cus can also be dehydrogenated by bridge O and OH species (i.e., by Obr and OHbr). We find that the barrier for N−H scission by Obr is 0.47 eV, which is smaller than that involving the Ocus pathway. However, this reaction has a very late TS, and the reverse barrier is only 0.09 eV, which is the smallest barrier in our database (Table 1). Since the reverse barrier is smaller than any other in the database, the reverse process is picked up whenever the forward process is executed. As a result, the net contribution of this reaction to N−H scission is negligible as confirmed by our earlier KMC simulations and in agreement with experimental findings that NH3 does not react on the stoichiometric RuO2(110) surface.3,6 Dehydrogenation of NH3 by OHbr (and OHcus) is not feasible owing to an unstable FS, as also pointed out by Wang et al.,7 whose calculated energy barriers for the above-mentioned processes are not too different from ours. NH2cus could also be dehydrogenated by bridge O and OH species. We find that the barrier for N−H scission by Obr is 0.54 eV in close agreement with Wang et al. (0.57 eV).7 However, as in the case of NH3cus + Obr, the reverse barrier is only 0.09 eV, the lowest barrier in our database (Table 1). Similarly, Wang et al.7 find it to be 0.16 eV. Thus, the contribution of this route to NH2 dehydrogenation can be ignored for the same reason. Dehydrogenation of NH2 by OHbr (and OHcus) is not feasible owing to unstable FS, as above. In contrast to the above processes, we find that dehydrogenation of NH by Obr is a spontaneous, nonactivated, exothermic reaction in agreement with a previous study.7 This is again initiated by H···Obr. Therefore, contribution of Obr-induced decomposition of NH species to Ncus formation is substantial and should be considered in KMC simulation. (3). NO and N2 Formation and Desorption. Our calculated IS, TS, and FS for NO formation are presented in Figure 7. The final product NO binds vertically to Rucus with the bond length d(Ru−N) of 1.84 Å and with the intramolecular bond length d(N−O) of 1.78 Å (see Table 5). The calculated energy barrier of N + O recombination is 0.18 eV (0.13 eV without zero-point correction), almost the same as in our earlier study5 (0.14 eV). (This time we used CI-NEB, whereas previously we used the

Figure 7. Geometries for NO formation. (a and b) Top and side view for IS. (c and d) Top and side view for TS. (e and f) Top and side view for FS. 5231



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Table 4. Comparison of Calculated Energy Barriers of Selected Reaction Stepsa work computational setup

reaction

a

code pseudopotential exchange-correlation energy cutoff (eV) surface unit cell trilayers k-point grid spin-polarization Ncus + Ocus → NOcus Ncus + Ncus → N2 NOcus + Ncus → NNOcus NNOcus → N2 + Ocus NOcus diffusion Ncus diffusion Ocus diffusion

this study

Wang et al.7

Seitsonen et al.6

Schneider et al.8

́ et al.4 Pérez-Ramirez

QE US PBE 408 (3 × 1) 3 3×6×1 YES 0.18 0.27 0.79 0.81 1.42 1.13 1.05

VASP PAW PW91 300 (2 × 1) 4 3×3×1 NO 0.47 0.20 0.85 1.35 1.44 -

VASP PAW PBE 503 5 4×8×1 NO 0.79 -

VASP PAW PW91 400 (3 × 1) 3 6×8×1 YES (partially) 0.7 0.8 1.22 1.8 -

VASP PAW RPBE 400 (4 × 1) 3 4×8×1 NO 1.01 1.11 1.30 0.31 1.67 ∼2.0 ∼2.0

The energy barriers reported in this study are all spin polarization and zero-point energy corrected.

not the cause for the large barrier since the rotation of the vertical N2O (Figure 10a) to horizontal N2O (Figure 10b) requires only 0.26 eV. In fact, we find that the large barrier could be an artifact of NEB calculations. We could reproduce the large energy barrier if we use NEB images generated by direct interpolation of IS (Figure 10a) and FS (Figure 10d). In such NEB routes the stiff N−O bond length is elongated, and thereby TS energy is increased. Our workaround for the problem is to use the horizontal N2O as IS. NEB then yields a much smaller barrier of 0.55 eV. Of course, the net barrier for the entire route of N2O decomposition is 0.81 eV (sum of 0.55 and 0.26 eV). ́ et al.4 report a remarkably On the other hand, Pérez-Ramirez small decomposition barrier of 0.26 eV, which is the energy barrier for desorption of horizontal, bidentate N2O. This species is similar to that in Figure 10 (e and f). However, our calculations, in agreement with those of Wang et al.,7 show that this horizontal species is unstable (i.e., is not an intermediate). ́ et al.4 do not report an energy barrier for the Pérez-Ramirez decomposition of vertical N2O to N2. (6). Diffusion Processes. We find a diffusion barrier of N, O, and NO from Rucus to the nearest-neighbor Rucus to be 1.13, 1.05, and 1.42 eV, respectively. The other studies find (much) higher diffusion barriers (see Table 4). The energy barrier for OH diffusion along the linear chain of Rucus is 0.91 eV. The TS for the diffusion of N, O, NO, and OH along the linear chain of Rucus is shown in Figure 11. For H diffusion along the linear chain of Obr it needs to overcome a large energy barrier of 2.23 eV, in agreement with Seitsonen et al.6 (2.48 eV6). Similarly, H diffusion from Obr to the nearest-neighboring OHbr, which leads to the formation of

Figure 8. Geometries for N2 formation. (a and b) Top and side view for IS. (c and d) Top and side view for TS. (e and f) Top and side view for FS.

Figure 9. Geometries for N2O formation. (a and b) Top and side view for transition state. (c and d) Top and side view of horizontal N2O which is intermediate. (e and f) Top and side view of the vertical final state.

Figure 10. Geometries for N2O decomposition. (a and b) Top and side view for the initial state. (c and d) Top and side view of the intermediate state. (e and f) Top and side view for the transition state. (g and h) Top and side view for the final state.

by 0.21 eV with the reverse energy barrier of 1.02 eV and is an important route for N2 formation on RuO2(110). In contrast, Wang et al.7 report a much larger N2O decomposition barrier (1.35 eV), which they attribute to a large separation of N2O from the RuO2(110) surface. In our view the large separation is

Figure 11. Geometries for TS of the diffusion processes. (a and b) Top and side view for N diffusion. (c and d) Top and side view for O diffusion. (e and f) Top and side view for NO diffusion. (g and h) Top and side view for OH diffusion along the linear chain of Rucus. 5232



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Table 5. Geometrical Parameters (in Å) of Various Species on the RuO2(110) Surface species NH3cus NH2cus NOcus N2cus N2Ocus H2Ocus

Wang et al.7,9

this study d(N−Ru) d(N−Ru) d(N−Ru) d(N−Ru) d(N−Ru) d(O−Ru)

= = = = = =

2.13 1.96 1.81 2.11 2.14 1.98

d(N−H) d(N−H) d(N−O) d(N−N) d(N−N) d(O−H)

= = = = = =

1.02, 1.02, 1.03 1.02, 1.02 1.16 1.11 1.14 0.98

d(N−Ru) d(N−Ru) d(N−Ru) d(N−Ru) d(N−Ru) d(O−Ru)

= = = = = =

2.16 1.93 1.78 2.02 2.06 2.16

d(N−H) d(N−H) d(N−O) d(N−N) d(N−N) d(O−H)

= = = = = =

1.03, 1.03, 1.04 1.03, 1.03 1.17 1.12 1.14 1.01

Figure 12. Plot of selectivity of N2, N2O, and NO formation on RuO2(110) in UHV, as a function of O2/NH3 partial pressure for selected temperatures, using our database.

Figure 13. Snapshot of RuO2(110) during NH3 oxidation reaction at UHV conditions for (left) O2/NH3 = 10 and T = 373 K and (right) O2/NH3 = 10 and T = 630 K (notation: · = empty site, 1 = NH3, 2 = NH2, 3 = NH, 4 = N, 5 = O, 6 = OH, 7 = H2O, 8 = NO).

is usually negligible for molecules adsorbed on RuO2(110) owing to spin-quenching, it cannot be ignored in the following sets of calculations: (1) total energy of gaseous molecules that have substantial spin polarization such as NO and O2 and (2) total energy of these molecules adsorbed on the surface in TS. In TS, since the molecule-to-surface bond length is elongated, spin-quenching of the molecules could be incomplete. In such situations spin-polarized total energy should be calculated. This is particularly important for case (1) above since the nonspinpolarized total energy of these gaseous molecules would contain substantial error, which may not cancel out in the calculations of the adsorption energy and energy barriers. For example, the nonspin-polarized adsorption energy of NO on RuO2(110) is 2.13 eV, but its spin-polarized value is 1.83 eV. Spin-polarization correction for reaction barriers, on the other hand, may be smaller: we find it to be in range of 40−90 meV (increase from nonspin-polarized energy barrier) for the

H2Obr species, has a large energy barrier of 2.21 eV with the reverse energy barrier of 1.20 eV. Thus, the formation of H2Obr via H diffusion along the linear chain of Obr is a rare event. This result indicates that O-vacancy formation on RuO2(110) during the reaction can be ignored. (7). Comparison among Available Theories. The discrepancies among the calculated energy barriers as discussed above and evident from the tables are remarkable but not new. In fact, a similar discrepancy was already reported for CO oxidation reaction on RuO2(110).13,22 As summarized in Table 4, there are differences in the details of computational parameters and calculational procedures used to evaluate adsorption energies and energy barriers, which contribute to some of the variations in the calculated system energetics. In this regard, we find that ignoring spin-polarization in calculations of adsorption energies and energy barriers can be an important source of error. While the spin-polarization effect 5233



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5, the percentage of decomposed N2O is only ∼1% of the total N2O population on RuO2(110). High selectivity of NO (>95%) at 630 K and beyond is the result of thermal activation of NOcus desorption. Since the NO formation barrier is only 0.18 eV, NO formation is active even at low temperatures. However, most of the NO remains at the surface at temperatures 5234



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Figure 15. Plot of selectivity of N2, N2O, and NO on RuO2(110) at UHV conditions as a function of O2/NH3 partial pressure for selected temperatures, obtained from Wang et al.’s database.7

Figure 16. (a) Selectivity and (b) reaction rates of N2, N2O, and NO on RuO2(110) at ambient condition as a function of O2/NH3 partial pressure for selected temperatures, obtained using our database.

surface facets of the polycrystalline sample. Since these experiments were performed for polycrystalline RuO2 samples, it is not easy to decouple surface-specific contributions to N2 selectivity. Here we present the results from our KMC simulations using three sets of databases (ours, Wang et al.’s,7 ́ et al.’s4) under ambient pressures. and Pérez-Ramirez a. KMC Results Using Our Database. KMC results for our database (Table 1) in Figure 16, in marked contrast to the UHV results, show that there is no dominant product at ambient conditions even at high temperatures >573 K, at which NO selectivity is nearly 100% under UHV conditions (Figure 12). From Figure 16a, we find that (1) N2 is a major product in all ranges investigated (indicating that N2 formation is active), but its selectivity more or less reduces as temperature rises; (2) N2O selectivity is minor, but it increases with temperature; and (3) NO selectivity is contained in the range of 26−43%. Unlike the situation in UHV, at ambient pressure direct NO desorption does not dominate even at higher temperatures (for example, 773 K). Instead, NO undergoes a secondary reaction to form N2O. Our finding that N2 is the major reaction product is in ́ et al.’s experimental qualitative agreement with Pérez-Ramirez observations,4 indicating that there is a pressure gap in ammonia oxidation reaction on RuO2(110). What is then the rationale for the pressure gap? Note that there are two pathways for N2 formation on RuO2(110): Ncus + Ncus recombination and N2O decomposition. Our KMC simulations show that Ncus + Ncus recombination is very active at ambient pressures and is the dominant route for N2 formation. Surprisingly, the contribution from the N2O

involving processes. Nevertheless, our present results predict a different product landscape from the earlier study in that N2O is a new product, which was absent in the earlier study. KMC results from Wang et al.’s database7 (Table 2) presented in Figure 15 show a trend similar to ours. As a result of differences in the calculated energy barriers for key processes, the temperatures at which the selectivity of a particular species is most pronounced are different from those in Figure 12. For example, their NO desorption barrier of 2.09 eV as compared to our 1.72 eV shifts the NO desorption peak to 750 K. Therefore, for comparison with results from our database (Figure 12), phase-to-phase comparison is more informative than direct temperature-to-temperature comparison. We see the same transition of the dominant product from N2 to N2O, to NO as temperature increases. Also, regarding participation of Obr in ammonia oxidation reaction on RuO2(110), their database shows that the contribution of Obr-induced dehydrogenation of NHx(x=2,3) species is effectively zero, but Obr-induced dehydrogenation of NH dehydrogenation is substantial, in agreement with our database results. (2). Ambient Pressure Results. In a recent study on ammonia oxidation on polycrystalline RuO2 samples, Péreź et al.4 showed that NO selectivity is low, below 10% at Ramirez ambient pressure, in marked contrast to the nearly 100% selectivity in the UHV case. They showed that N2 is in fact the dominant product with more than 80% selectivity, and thus there is a pressure gap in ammonia oxidation by RuO2 catalyst. We, however, think that the high selectivity of N2 could indicate a material gap as well if the selectivity strongly depends on the 5235



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Figure 17. Coverage of NH3, N, O, OH, and NO on RuO2(110) under ambient conditions as a function of O2/NH3 for selected temperatures calculated using our database.

Figure 18. Selectivity of N2, N2O, and NO on RuO2(110) under ambient conditions as a function of O2/NH3 partial pressure for selected temperatures, for Wang et al.’s database.7

Figure 19. Selectivity of N2, N2O, and NO at (a) UHV and (b) ambient conditions for RuO2(110) and (c) ambient conditions for the RuO2(101) ́ et al.’s database. surface as a function of O2/NH3 partial pressure for selected temperatures, obtained using Pérez-Ramirez

conditions the probability for occurrence of the NO + N configuration is quite low since there are not many N species on the surface. As a result, most NO directly desorbs, not undergoing secondary reactions. In contrast, at ambient pressures, as NH3 conversion to N is much more (by orders of magnitude) active than in UHV conditions, the probability of NO to meet N is much higher, leading to an increase in NO + N recombination. This is the rationale for the increase in N2O formation with increasing temperature in Figure 16b and consequent reduction of N2 and NO selectivity (Figure 16a and 16b). In sum, under ambient conditions, the higher pressure (by 8 orders of magnitude) as compared to UHV brings about active dehydrogenation of NH3 to N, which increases the probability for N or NO species to meet one another, with the result that N + N and NO + N recombination rates increase by orders of magnitude even at temperatures >673 K. b. KMC Results Using Wang et al.’s Database. The KMC results from Wang et al.’s database7 (Table 2) in Figure 18 exhibit trends similar to those found from our database. However, there are remarkable differences in the results as compared to that from our database: (1) N2 (and NO) are only minor products at temperatures >573 K, and (2) N2O is the dominant product at temperatures >573 K. The high selectivity of N2O (and the small selectivity of NO below 20%) in their database is a result of the competition of NO desorption and NO + N recombination for N2O

decomposition route is negligible (less than 1%) even at high temperatures. We present a calculated steady-state reaction rate for N2 formation in Figure 16b and surface coverage for NH3, N, O, OH, and NO on RuO2(110) in Figure 17. The reaction rate of N2 formation substantially increases with O2/NH3 ratio and by several orders of magnitude with temperature >573 K suggesting that N + N recombination is more active with larger O2/NH3 ratio and with higher temperature. The increase in N + N recombination rate is correlated with the change in NH3 coverage in Figure 17. Our KMC simulations show that the net NH3 influx (difference between the number of adsorbed and desorbed NH3 per site per unit time) to surface increases 24 times from 573 to 673 K and 9 times from 673 to 773 K for O2/NH3 = 10. Nevertheless, NH3 coverage hardly increases between 573 and 673 K and in fact even decreases from 673 to 773 K. Therefore, the trend in NH3 coverage in Figure 17 indicates that more NH3 converts to N at higher temperatures. As the supply of Ncus species becomes abundant as a result of active decomposition of NHx under ambient conditions, N + N recombination becomes active. Another outcome of the abundant supply of N species in ambient pressure is activation of NO + N recombination (N2O formation). Whenever NO and N adsorb nearby, they recombine to form N 2 O. (Note that the NO + N recombination barrier, which is 0.71 eV, is much smaller than the NO desorption barrier of 1.72 eV.) However, under UHV 5236



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d. Bridging the Gap between Theory and Experiment. As discussed earlier, although our ambient results are in qualitative ́ et al.,4 our agreement with the experiment of Pérez-Ramirez predicted N2 selectivity is at best 56%, which is much lower than the experimental N2 selectivity of more than 80%. N2 selectivity is affected by three reactions: (1) N + N → N2, (2) NO + N → N2O, and (3) N2O → N2 + O. As we discussed earlier, (1) is active under ambient conditions and responsible for the 56% selectivity. Therefore, the discrepancy between experiment and theory must be related to (2) and (3). Indeed, while both our and Wang et al.’s7 databases predict that (3) is very slow and has a negligible contribution to N2 selectivity, ́ et al.4 conclude that facile N2O decomposition Pérez-Ramirez to N2 is the rationale for the high N2 selectivity under ambient conditions. In both theories the negligible contribution of N2O decomposition to N2 selectivity was a result of its competition with direct desorption of N2O; i.e., the majority of N2Ocus desorbs rather than decomposes to N2. (In our database, the former is activated by 0.81 eV and the latter by 0.47 eV.) Nevertheless, in an attempt to bridge the gap with experiment, if we set hypothetically the decomposition barrier to a much smaller one, say 0.31 eV, as reported by Péreź et al.,4 our KMC simulations predict that nearly half of Ramirez N2Ocus would decompose to N2. The impact of this adjustment on N2 selectivity is not so remarkable since the new N2 selectivity is only about ∼54% at 773 K. Although it is 18% up from the unadjusted selectivity, it still falls short of experimental selectivity (∼73%) at the temperature. If we hypothesize even further that the perfect conversion of N2O to N2 occurs on RuO2(110), then our KMC would predict N2 selectivity of 60−75% close to the experimental selectivity (70− 95%). In the case of Wang et al.’s database,7 the hypothesized perfect conversion would lead to N2 selectivity of more than 80% up to 100% also in good agreement with the Péreź et al.’s experiment.4 Thus, perfect N2O decomposition Ramirez to N2 (if possible) would lead to good agreement with experiment for both databases. However, such a hypothesis is not in accord with both our and Wang et al.’s7 DFT structure and energetics calculations, as discussed in Section III.A.5. Finally, we speculate that the discrepancy could be attributed ́ et al.4 used polycrystalto a material gap since Pérez-Ramirez line RuO2 particles in their experiments, which consist of several facets including (110), (101), and (211). Since these facets may have low coordinated sites such as step and corner atoms at the boundaries, NH3 dehydrogenation and N2O decomposition reactions occurring at these sites could be activated with smaller energy barriers, which would eventually increase N formation rate and thus N2 selectivity.

formation. The former is activated by 2.09 eV and the latter by 0.85 eV. Owing to such a big difference in activation energy, the latter is favored so that the majority of NOcus converts to N2Ocus. Thus-formed N2O desorbs instead of decomposing to N2. ́ et al.’s Database. In c. KMC Results Using Peŕ ez-Ramirez ́ et al.’s presenting the KMC results obtained for Pérez-Ramirez databases4 (Table 3) in Figure 19 we would like to reemphasize that these databases do not include NHx decomposition processes (Table 3). Rather, their database contains only secondary reactions of N and O. Therefore, KMC simulations of their database simulate only a part of the NH3 oxidation reaction network on RuO2(110). We varied O/N ratio from 0.5 to 20 for two fixed N pressures: p(N) = 10−7 (named “UHV” ́ case) and 101 mbar (named “AMB” case). Since Pérez-Ramirez et al.4 reported two databases: one for RuO2(110) and one for RuO2(101), we present UHV and ambient results for the former in Figure 19a and 19b, respectively, and ambient results for the latter in Figure 19c. The two surface-specific databases ́ et al.4 give almost identical results (compare of Pérez-Ramirez Figures 19b and 19c), although, as will be discussed later, this does not necessarily indicate that there is no material gap since their polycrystalline RuO2 samples contain various facets such as (211) and (110) and (101). The most surprising result is, however, NO dominance in nearly all temperature and pressure ranges regardless of UHV or ambient pressures (compare Figures 19a and 19b). Thus, the KMC simulations of their databases indicate no pressure gap, which is in disagreement with their experiment and other studies! On the one hand, this may simply reflect the incompleteness of their database. Further analysis of the KMC results for their database reveals the reason for the failure: at O/N ratio >3, the surface O species poisons the secondary reaction of NOcus (NOcus + Ncus). (This happens regardless of the magnitude of absolute pressure of N and O.) As a result, the formation rate of N2O species effectively reduces by a factor of 2 when O/N varies from 2 to 5, and so does N2 formation. This explains why N2 selectivity drops quickly as O/N increases. Since diffusion barriers for Ocus, Ncus, NOcus, and other surface species are very high, they do not diffuse but occupy their adsorption sites permanently. Therefore, reaction will not happen unless “proper” reactants adsorb next to them. In the case that an “improper” species adsorbs next to them, their sites remain blocked until these species are removed. In such a situation, N + N and N + O recombination are the only reactions that are likely to occur with high probability. This explains the missing of N2O in their product. In summary, the present study is a good example that shows the pitfalls of estimation of reactivity (selectivity) solely on the ́ et al.4 basis of energetics. Since the database of Pérez-Ramirez provides a low-energy barrier for the N2O decomposition process (only 0.31 eV, the lowest energy barrier process in the database) it is easy to draw the conclusion that N2 is the dominant product because of facile conversion of N2O to N2. ́ et al.4 (This was in fact the conclusion drawn by Pérez-Ramirez on the basis of the 12 processes in their database.) However, reactions on the catalytic surface are affected by the local environment such as surface coverage, diffusion of reactants, and, most of all, blocking of active sites. It is thus important that local interactions between surface species as well as the competition between direct and indirect reactions be included in the consideration in addition to the energetics of key reaction steps.

IV. CONCLUSIONS We have performed KMC simulations in ambient conditions for ammonia oxidation in the RuO2(110) surface using ́ et available databases, which were proposed by Pérez-Ramirez al.4 and Wang et al.,7 as well as our own database of 24 processes. First of all, KMC results for all databases show that NO is the dominant product in UHV at or above the peak NO desorption temperature, confirming related experimental results.3 In contrast, ambient KMC results using our database show that N2 is a major product, also in qualitative agreement with experiment under similar conditions.4 Thus, our KMC simulations confirm a pressure gap in the ammonia oxidation reaction on the RuO2(110) surface. The rationale for the pressure gap is the active recombination of Ncus + Ncus owing to 5237



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(16) Paolo, G.; Stefano, B.; Nicola, B.; Matteo, C.; Roberto, C.; Carlo, C.; Davide, C.; Guido, L. C.; Matteo, C.; Ismaila, D.; et al. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (17) Truhlar, D. G.; Garrett, B. C.; Klippenstein, S. J. Current Status of Transition-State Theory. J. Phys. Chem. 1996, 100, 12771−12800. (18) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (19) Vineyard, G. H. Frequency Factors and Isotope Effects in Solid State Rate Processes. J. Phys. Chem. Solids 1957, 3, 121−127. (20) Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203−211. (21) Gorte, R. J.; Schmidt, L. D.; Gland, J. L. Binding States and Decomposition of NO on Single Crystal Planes of Pt. Surf. Sci. 1981, 109, 367−380. (22) Kiejna, A.; Kresse, G.; Rogal, J.; De Sarkar, A.; Reuter, K.; Scheffler, M. Comparison of the Full-Potential and Frozen-Core Approximation Approaches to Density-Functional Calculations of Surfaces. Phys. Rev. B 2006, 73, 035404. (23) Antony, A.; Asthagiri, A.; Weaver, J. F. Pathways for C-H Bond Cleavage of Propane [Sigma]-Complexes on Pdo(101). Phys. Chem. Chem. Phys. 2012, 14, 12202−12212. (24) Boudart, M.; Löffler, D. G. Rate of Adsorption to and Desorption from a Langmuir Surface: The Case of Zero Activation Barrier to Adsorption without Dissociation. Catal. Lett. 1990, 6, 317− 320.

the abundant supply of N species at ambient pressures as a result of active NHx decomposition by plenty of O species that exist on the RuO2(110) surface. Contribution of N2O decomposition to N2 selectivity is negligible. Finally, we show that simple estimation of selectivity based solely on the energetics of some key processes could be unreliable.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Department of Energy, Basic Energy Sciences (DE-FG02-07ER15842). The DFT calculations were performed using the computing resources at the National Energy Research Scientific Computing Center (NERSC), the Center for Nanoscale Materials (CNM) of the Argonne National Laboratory, and at STOKES, the highperformance computational facility at UCF.

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REFERENCES

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Combined Density Functional Theory and Kinetic Monte Carlo Study

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