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Oct 9, 2015 - ABSTRACT: The mechanisms of NO reduction by H2 on the Pt(100) surface and the surface modified with subsurface oxygen atoms ...
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Effect of Subsurface Oxygen on Selective Catalytic Reduction of NO by H2 on Pt(100): A First-Principles Study Li-yuan Huai, Hui Wang, Chao-zheng He, Hong Wen, Wen-cai Yi, and Jing-yao Liu* Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: The mechanisms of NO reduction by H2 on the Pt(100) surface and the surface modified with subsurface oxygen atoms (Md-Pt(100)) are studied by firstprinciples calculations. Similar catalytic activity toward NO dissociation is found on both surfaces with barriers of 0.86 and 0.96 eV, respectively. The pathway of N + N → N2 rather than NO + N → N2 + O is the N2 formation pathway on the Pt(100) surface, while these two pathways are competitive on the Md-Pt(100) surface. The NH3 formation is almost negligible, and reductant hydrogen can effectively remove the surface oxygen on both surfaces. The microkinetic analysis further confirms that, compared to the high selectivity toward N2O (almost 100% at 300−500 K) on the clean surface, higher N2 lowtemperature selectivity (larger than 90%) is achieved on the Md-Pt(100) surface under lower pressure. The present study shows that subsurface oxygen has an enhanced effect for improving the N2 selectivity of NO reduction on Pt catalysts.

1. INTRODUCTION Over the past decades, the catalytic elimination of nitrogen oxide (NO) from automobile exhaust gases and industrial combustion of fossil fuels has attracted long-standing attention because of its harmful impact on the environment and people’s health.1 The current three-way catalysts (TWCs) including Pt, Pd, and Rh are widely used to catalytically convert NO to benign N2.2−5 However, the remaining oxygen atoms on metal surfaces poison NO dissociation sites, thereby resulting in the deactivation of catalysts. In particular, for diesel and lean-burn gasoline engines, TWCs are not suitable for the effective removal of NO. The selective catalytic reduction (SCR) scheme employing CO, HCx, NH3, and H2 as reductants is one of the promising technologies to solve this problem.6−17 The H2-SCR technology has aroused much attention in recent years, not only because H2 is easily available in the exhaust stream, but also because its combustion oxidation product, H2O, makes it environmentally friendly. Many reports show that reductant H2 exhibits high reactivity and selectivity on noble-metal-based catalysts under lean-burn conditions, with reduced operation temperatures.1,9−20 Among them, the Pt catalysts for NO-SCRs have proved to be highly active at relatively low temperatures, and the presence of reducing agent H2 is more efficient than CO on the Pt(100) surface.18 However, one major problem of Pt catalysts is the high selectivity for the undesired byproduct N2O.15,16,19,20 Therefore, in order to improve the N2 selectivity on Pt catalysts at low temperature, a better understanding of the catalytic processes is very necessary. However, the knowledge of the reaction mechanism of NO reduction by H2 on Pt surface is very limited. In addition, it has been well-known that oxygen can penetrate the surface under reaction conditions, which will modify the surface structure. A number of reports show that the © 2015 American Chemical Society

subsurface oxygen plays a role in changing the adsorption characteristics and reaction catalytic activity such as on Pd, Au, Ag, and Pt catalysts.13,21−26 For example, the subsurface oxygen modified Pd(111) surface13 exhibits higher activity for H2-SCR of NO at near-ambient temperatures (325 K) and equally high selectivity toward N2 compared to the clean Pd(111) surface. The effect of subsurface oxygen on the reactivity of the Ag(111) surface was calculated, and the results indicate that subsurface oxygen greatly facilitates the dissociation of H2, O2, and NO.23 Similarly, the adsorption of CO and the adsorption of O2 are also stabilized in the presence of subsurface oxygen on Ag(111) and the surface reactivity is enhanced.21 The formation of subsurface oxygen on the Pt(100) surface was also observed above 450 K under a low pressure of 5 × 10−6 mbar (3.74 × 10−6 Torr)27 and above 480 K under an intermediate pressure of 6 × 10−2 Torr.25 The experiment in ref 25 showed that the subsurface oxygen is located directly underneath the first atomic layer of the surface and could persist relatively unchanged for long times (minutes to hours) after formation, even during reaction conditions at intermediate pressure and higher temperatures. The desorption of subsurface oxygen occurred at temperatures between 650 and 770 K. The effect of subsurface oxygen toward CO oxidation was similar to that on the Ag(111) surface. However, the effect of subsurface oxygen toward NO reduction on the Pt(100) surface has not been studied yet. Considering the importance of subsurface oxygen in catalytic reactions and surface reconstruction, it is highly desirable to clarify the role of subsurface oxygen for NO Received: July 24, 2015 Revised: October 9, 2015 Published: October 9, 2015 24819

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energy was obtained by the energy difference between the product(s) and reactant(s) at their respective most stable positions or infinite states.

reduction by H2 on the Pt(100) surface under oxygen-rich conditions. In the present work, we performed a first-principles study based on density functional theory (DFT) calculations to address the mechanism of H2-SCR of NO on the Pt(100) surface and the surface modified with oxygen atoms in the subsurface in detail. The influence of reducing agent H2 and subsurface oxygen on the reaction activity and selectivity are discussed. A microkinetic model was established to analyze the reaction rate and the possible selectivity for N-containing products on both surfaces. We aim to provide a comprehensive understanding of the mechanism of NO reduction by H2 and some insights into the fundamental issues concerning the activity and selectivity of Pt-based catalysts.

3. RESULTS AND DISCUSSION Four different coverages of oxygen atoms predosed at the subsurface of the Pt(100) surface were considered in order to identify a proper model. The oxygen prefers the tetrahedral site, and the calculated average adsorption energies of oxygen atoms are −2.39, −3.01, −2.86, and −2.89 eV at 0.25, 0.50, 0.75, and 1.0 ML, respectively. This method of calculating the oxygen adsorption capacity on the subsurface is also reported in the previous literature.21,23,26 The calculated value is much less than that of on-surface oxygen by about 1.41 eV, but the adsorption of subsurface oxygen species are thermodynamically stable. Hence, it is feasible to penetrate the oxygen into the subsurface under appropriate experimental conditions such as that on the Pd(111) surface.13 Taking into account the surface symmetry and the adsorption stability of the penetrated oxygen atoms, we selected the 0.50 ML predosed oxygen atoms at the subsurface of Pt(100) as the modified surface. The top and side views of Pt(100) and Md-Pt(100) surfaces are presented in Figure 1. In

2. THEORETICAL METHODS We performed all the spin-polarized periodic DFT calculations with the Vienna Ab initio Simulation Package (VASP).28,29 The projector augmented wave (PAW) pseudopotentials were employed to describe the electron−ion interactions.30,31 The exchange-correlation effects were treated by the Perdew and Wang (PW91) functional32 with the generalized gradient approximation (GGA). The energy cutoff for the plane-wave basis sets was set to be 400 eV. The Brillouin zones were sampled with the Monkhorst−Pack k-points33 of 15 × 15 × 15 for bulk calculation. Our calculated equilibrium lattice constant of 3.98 Å is in good agreement with the previous theoretical (3.99 Å)34 and experimental (3.92 Å)35 results. The Pt(100) surface was simulated by 2 × 2 unit cells with five periodic atom layers, and a vacuum thickness of 15 Å was used to remove any interactions between slabs. The oxygen modified Pt(100) surface was built with 0.50 monolayer (ML) of predosed oxygen atoms at the subsurface. In both surfaces, the bottom two Pt layers were fixed and the top three layers were fully relaxed. The calculations stopped when the forces were less than 0.02 eV/Å and the dipole moment correction along the z direction was applied. The Brillouin zone sampling was carried out using the Monkhorst−Pack k-point of 7 × 7 × 1. The transition states (TSs) were well located with the climbing-image nudged elastic band (CI-NEB) method36,37 and verified by the existence of a single imaginary frequency based on the vibrational analysis. All possible reaction pathways for each reaction were considered, but only the reaction path with the lowest energy barrier was reported. Zero-point-energy (ZPE) corrections were considered during all the calculations. The adsorption energies (Eads) for the adsorbates were calculated according to Eads = Eadsorbate − surface − Esurface − Eadsorbate

Figure 1. Top (a1, a2) and side (b1, b2) views of Pt(100) and MdPt(100) surfaces, respectively. Three possible adsorption sites of hollow, bridge, and top sites were labeled as H, B, and T, respectively, for Pt(100) surface. Eight possible adsorption sites of hollow, bridge, and top sites were labeled as H1, H2, B1, B2, B3, B4, T1, and T2, respectively, for oxygen modified Pt(100) surface.

where Eadsorbate, Esurface, and Eadsorbate−surface represent the energies of the free adsorbate in gas phase, the corresponding surface (clean Pt(100) surface or the surface modified with subsurface oxygen atoms (Md-Pt(100)), and the surface with the adsorbate, respectively. The interaction energy was defined as the energy difference between the coadsorbed configuration (A*···B*) and their infinite states (A* + B*). In their infinite states, the adsorbed two species A* and B* do not interact with each other, that is, they are separated by infinite distance, and each of them lies at its most stable position in one unit cell. Unless otherwise specified, the reaction energy barrier for each elementary step was calculated as the energy difference between the transition state and the reactants at their infinite states (or one reactant at its most stable position); similarly, the reaction

the Md-Pt(100) surface, the most stable surface is where the oxygen atoms penetrate at their most stable tetrahedral site in a row in the subsurface. The interlayer distances between the first and second Pt atoms are raised about 1.23 and 0.32 Å, respectively, compared to that on the clean Pt(100) surface. The corresponding bond lengths between the Pt atoms at the first layer and subsurface oxygen (Pt−O) are 2.00 and 2.06 Å, respectively. There are three different adsorption sites for the clean Pt(100) surface: top (T), bridge (B), and hollow (H). There are eight different sites for the Md-Pt(100) surface: two 24820

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adsorbed on the surface with one H atom at the T1 site and the other H atom at the B2 site. The single H atom prefers to reside at the T1 site with the adsorption energy of −2.95 eV. 3.1.3. Adsorption of OH and H2O. On the Pt(100) surface, the OH intermediate was most stable at the bridge site with the O atom binding to two Pt atoms and the H atom pointing to the hollow site. The calculated adsorption energy is −2.83 eV. H2O binds to the top site parallel to the surface through the O atom with the adsorption energy of −0.31 eV. On the MdPt(100) surface, the OH intermediate prefers the B3 site and H2O prefers the T1 site, with adsorption energies of −3.11 and −0.72 eV, respectively. 3.1.4. Adsorption of NH, NH2, and NH3. On the Pt(100) surface, the adsorption of NH intermediate at the hollow site is most favorable, NH2 prefers the bridge site, and NH3 is only stable at the top site, while on the Md-Pt(100) surface, both NH and NH2 prefer the B3 site and NH3 adsorbs only at the T1 site. The adsorption energies for these three intermediates are −4.01, −3.01, and −0.78 eV on the Pt(100) surface, respectively, and −3.78, −3.14, and −1.20 eV on the MdPt(100) surface, respectively. 3.2. Elementary Steps on Clean and Oxygen Modified Pt(100) Surfaces. 3.2.1. NO Dissociation. As shown in Figure 2, on the clean Pt(100) surface, the NO direct dissociation comes from the most stable bridge site to form N and O atoms via the transition state TS1. This step has an energy barrier of 0.86 eV and is endothermic by 0.59 eV. The result shows that NO dissociation activity on the clean Pt(100) surface is much better than that on Pd(111)5,38 and Rh(111)5 surfaces. On the Md-Pt(100) surface, the favorite pathway for NO dissociation is from the bridge (B2) site with the N−O bond vertical to the surface and the bond length of 1.20 Å, which is the same as that (1.20 Å) at the bridge site of the Pt(100) surface. This step needs to overcome an energy barrier of 0.96 eV to produce N and O atoms at the B2 sites and is endothermic by 0.46 eV relative to the NO at its most stable adsorption B3 site. The results imply similar activity of NO dissociation on both surfaces. Apart from the NO direct dissociation, the reaction of two coadsorbed NO molecules to form N2O and O was also considered. It is found that no (NO)2 dimer can be formed on both surfaces. On the Pt(100) surface, the initial state of two coadsorbed NO molecules and the final state of coadsorbed N2O and O are all at the bridge site. This step needs to overcome an energy barrier of 2.46 eV and is endothermic by 1.36 eV. On the Md-Pt(100) surface, the favorable pathway starts from two coadsorbed NO molecules at the adjacent B2 site and finishes with the coadsorbed state of N2O and O at the

top sites (T1 and T2), four bridge sites (B1, B2, B3, and B4), and two hollow sites (H1 and H2). The geometric and energetic parameters of reaction intermediates at the most stable sites are compiled in Table 1. Table 1. Geometric and Energetic Parameters of Reaction Intermediates As Identified in the Stable State Pt(100) reaction intermediates NO N O N2O N2 H NH NH2 NH3 OH H2O

Md-Pt(100)

configuration

ads energy (eV)

configuration

ads energy (eV)

bridge hollow bridge hollow top bridge hollow bridge top bridge top

−2.25 −4.60 −4.43 −0.25 −0.37 −2.79 −4.01 −3.01 −0.78 −2.83 −0.31

bridge (B3) bridge (B3) bridge (B3) bridge (B3) top (T1) top (T1) bridge (B3) bridge (B3) top (T1) bridge (B3) top (T1)

−2.15 −4.32 −4.57 −0.67 −0.78 −2.95 −3.78 −3.14 −1.20 −3.11 −0.72

3.1. Adsorption of Intermediates on Clean Pt(100) and Md-Pt(100) Surfaces. 3.1.1. Adsorption of NxOy (x = 0− 2, y = 0−1). On clean Pt(100) surface, NO and O prefer to adsorb at the bridge site with adsorption energies of −2.25 and −4.43 eV, respectively. The most stable adsorption site for N atom and N2O molecule is the hollow site with adsorption energies of −4.60 and −0.25 eV, respectively. N2 is favored to reside at the top site with an adsorption energy of −0.37 eV. The very low adsorption energies of N2O and N2 indicate that they are easily desorbed from the surface when they are formed. On the Md-Pt(100) surface, NO, N, O, and N2O preferentially occupy the bridge (B3) site with adsorption energies of −2.15, −4.32, −4.57, and −0.67 eV, respectively. Upon NO adsorption, the N−O bond length is almost the same on both surfaces (from 1.17 Å at gas phase to 1.20 Å), which means similar NO dissociation activity. The lengths between surface Pt atoms and subsurface oxygen (Pt−O) are changed from 2.00 to 2.10 Å and from 2.06 Å to 2.07 Å, respectively, implying the small distortion of the surface induced by adsorption. The most stable configuration for the N2 molecule is at the top (T1) site with the adsorption energy of −0.78 eV. 3.1.2. Adsorption of H2 and H. On the clean Pt(100) surface, H2 was found physically adsorbing at the Pt(100) surface with an adsorption energy of only −0.02 eV. The H atom prefers the bridge site with the adsorption energy of −2.79 eV. On the Md-Pt(100) surface, H2 is dissociatively

Figure 2. Pathways for NO dissociation on clean Pt(100) and Md-Pt(100) surfaces. 24821

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initial state, which is different from the case on the clean surface. The reaction energy barrier is reduced to 0.58 eV, and the reaction is more exothermic with a value of 1.73 eV. 3.2.3. Formation Mechanism for N2O. The combination reaction of N atom with NO molecule to form N2O on both surfaces is shown in Figure 4. On the clean Pt(100) surface, the

B3 and B4 sites, respectively. The energy barrier for this step is 2.13 eV and the reaction energy is 0.61 eV. Therefore, the reaction of NO + NO → N2O + O is unfavorable on both surfaces. We also considered the possible pathways of NO dissociation assisted by predosed H atom on clean and Md-Pt(100) surfaces; i.e., the surface H atom serves as a spectator; NO hydrogenates to intermediate NOH before dissociation; NO hydrogenates to intermediate HNO before dissociation. The calculated energy barriers for these three steps on the Pt(100) surface are 1.28, 1.18, and 2.45 eV, respectively. These results show that the preadsorbed H atom highly inhibits the NO dissociation. Similarly, on the Md-Pt(100) surface, the energy barriers for these three pathways are 1.41, 1.74, and 1.23 eV, respectively. 3.2.2. Formation Mechanism for N2. 3.2.2.1. Recombination Reaction Pathway (N + N). On the clean Pt(100) surface (see Figure 3), starting from the most stable coadsorbed initial

Figure 4. Potential energy diagram and transition state structures for N2O formation on clean Pt(100) and Md-Pt(100) surfaces.

reaction starts from the coadsorbed configuration of both N and NO at the bridge sites with a repulsive interaction energy of 0.39 eV and finishes with N2O situating at the hollow site through its N−N bond aligning parallel to the surface (via TS4). The calculated energy barrier is 0.84 eV, and the reaction is endothermic by 0.77 eV. On the Md-Pt(100) surface, the initial state for this step is both NO and N adsorbed at the B2 site with a repulsive energy of 0.33 eV and the final state is N2O adsorbed at the B3 site with an adsorption energy of −0.67 eV. The energy barrier of this step is 0.76 eV. 3.2.4. Formation Mechanism for NH3. NH3 formation involves the successive hydrogenation reactions of N atom as depicted in Figure 5. On the clean Pt(100) surface, the hydrogenation reactions of NHx (x = 0−2) start from the coadsorbed initial states of both NHx and H at the bridge site and finish with NH, NH2, and NH3 adsorbing at the hollow, bridge, and top sites, respectively. The calculated energy barriers for these three steps are 0.73, 0.81, and 1.35 eV, and their reaction energies are −0.21, −0.23, and 0.44 eV, respectively. On the Md-Pt(100) surface, the initial states of coadsorbed NHx and H are at the adjacent B2 sites and the final states of NH, NH2, and NH3 are at the B2, B2, and T1 sites, respectively. These steps require overcoming the energy barriers of 0.77, 0.76, and 0.98 eV, respectively, and are exothermic by −0.09, −0.44, and 0.31 eV, respectively. Clearly, the rate-limiting step for NH3 formation on both surfaces is the step of NH2 + H → NH3. As illustrated above, since the energy barrier for NH formation is slightly larger than those for N2O and N2 formation on both surfaces, the other possible elementary reactions between NO or N and NH intermediate such as NO + NH → N2O + H, NO + NH → N2H + O, and NH + N → N2 + H were also considered. The energy barriers for these three reactions are 1.71, 1.73, and 1.85 eV on the clean Pt(100) surface, and 1.66, 0.91, and 1.98 eV on the Md-Pt(100) surface, respectively. The dissociation of N2H to N2 (N2H → N2 + H) has an energy barrier of 1.36 eV on Md-Pt(100). The above results indicate that these elementary steps are unfavorable and cannot be competitive with the NO direct dissociation or NH hydrogenation reaction. The higher energy barriers for the reactions between NO or N and NH also imply that further

Figure 3. Potential energy diagram and transition state structures for N2 formation on clean Pt(100) and Md-Pt(100) surfaces.

state (IS) of two N atoms adsorbed at the bridge site with a repulsive interaction energy of 0.61 eV, the recombination reaction readily occurs via TS2 by overcoming an energy barrier of 0.67 eV with a large exothermicity of 1.40 eV. The N−N distance at the TS2 is shortened about 0.55 Å compared to that at initial state. The desorption of N2 takes place easily since its adsorption energy is only −0.03 eV at the final state. On the Md-Pt(100) surface, the reaction starts from the initial state where the two N atoms are coadsorbed at the B2 sites with a repulsive energy of 0.44 eV, and then passes through the transition state TS2′ by overcoming an energy barrier of 0.50 eV. This reaction is exothermic by 2.36 eV. In TS2′, the N−N bond length is shortened about 0.60 Å relative to the initial state. The formed N2 is adsorbed at the H2 hollow site and is easy to desorb due to the weak adsorption energy of −0.42 eV. 3.2.2.2. Abstraction Reaction Pathway (N + NO). N2 formation might also arise from the direct abstraction reaction pathway (N + NO → N2 + O) (see Figure 3). On the clean Pt(100) surface, the coadsorbed state of NO and N at the adjacent bridge site as initial state is found to have a repulsive interaction energy of 0.39 eV. The reaction proceeds via Nabstraction transition state TS3 to produce N2 and coadsorbed O atom. In TS3, the N−O bond length is slightly elongated by 0.23 Å. This step needs to overcome an energy barrier of 1.69 eV and is exothermic by 0.82 eV. On the Md-Pt(100) surface, the initial state of coadsorbed NO and N at the B2 site has an repulsive energy of 0.33 eV, and at the transition state TS3′, the N−N bond length is largely shortened by 1.57 Å relative to the 24822

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Figure 5. Potential energy diagram and transition state structures for NH3 formation on clean Pt(100) and Md-Pt(100) surfaces.

Figure 6. Potential energy diagram and transition state structures for H2O formation on clean Pt(100) and Md-Pt(100) surfaces.

barriers are 0.63 and 0.60 eV, and the reaction energies are −0.29 and 0.25 eV, respectively. Similarly, we also considered the disproportion reaction for H2O formation, but the optimization for the coadsorbed configuration of product H2OT1···OB2 failed; instead, it prefers to return to the reactant state with the coadsorption configuration OHB2···OHB2 on the Md-Pt(100) surface. This may be due to the strong hydrogen bonding O···H between the adsorbed oxygen and the hydrogen of H2O on the Md-Pt(100) surface. 3.3. Comparison of the Mechanism and Reactivity on Both Surfaces. The NO direct dissociation reaction is the most favorable pathway for N−O bond scission on both surfaces with similar energy barriers of 0.86 and 0.96 eV, respectively, and the presence of H2 inhibits the NO dissociation on both surfaces. For the two reaction pathways of N2 formation on clean the Pt(100) surface, the nitrogen combination reaction (N + N → N2) is more favored with an energy barrier of 0.67 eV, while the N-abstraction reaction (N + NO → N2 + O) is kinetically unfavorable due to a larger energy barrier of 1.69 eV. It is shown that for N-containing products on the clean Pt(100) surface, the energy barrier for N2 formation is slightly lower (0.17 eV) than that for N2O formation, while NH3 formation is not competitive due to a higher rate limiting energy barrier (1.35 eV). However, the cases are different on the subsurface oxygen modified Pt(100) surface. The activities for N-containing product formation are improved. For the two pathways of N2 formation, the formation energy barriers are lower than that for N2O formation, and the NH3 formation is still not competitive with the rate limiting energy barrier of 0.98 eV. Clearly, N2 becomes the most favorable product on the modified surface. The above results

consideration for the more complicated reactions between NO or N and NHx (x = 2, 3) intermediate is not necessary. 3.2.5. Formation Mechanism for H2O. Figure 6 presents the possible pathways of H2O formation on clean Pt(100) and MdPt(100) surfaces. There are two possible pathways on the Pt(100) surface. (i) The first is the reaction pathway of successive hydrogenation of O atom, i.e., O + H → OH and OH + H → H2O. The initial states of the coadsorbed OHx (x = 0, 1) and H are at the adjacent bridge sites, and the final states of OH and H2O are at the bridge and top sites, respectively. The successive hydrogenation reactions proceed through the transition states TS8 and TS9. The energy barriers for these two elementary steps are calculated to be 0.40 and 0.82 eV, respectively, and the corresponding reaction energies are −0.31 and 0.23 eV, respectively. It is seen that the OH intermediate is easily generated on the Pt(100) surface, better than that on V(100),39 Ir(110),40 Ir(111),41 and Pd(111)38 surfaces. Therefore, we discussed (ii) the pathway of the disproportionation reaction of OH intermediate, i.e., OH + OH → H2O + O. The coadsorbed configuration of two OH intermediates at the bridge sites is the initial state, and the reaction is finished with H2O at the top site and O at the bridge site. This is an endothermic reaction, and the energy of the transition state is less than that at the final state after ZPE correction. The calculated energy barrier for this reaction is 0.04 eV (0.35 eV relative to the coadsorbed state), and the reaction energy is 0.54 eV. On the Md-Pt(100) surface, for the successive hydrogenation reactions, the coadsorption configurations of initial states OB3··· HB4 and OHB2···HT1 are less stable by 0.44 and 0.04 eV with respect to the infinite states, respectively. The calculated energy 24823

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The Journal of Physical Chemistry C indicate that the subsurface oxygen modified Pt(100) surface may possess good selectivity toward N2 compared to the clean Pt(100) surface. The produced surface oxygen is easily hydrogenated to produce H2O on both surfaces. There are two competitive pathways for H2O formation, with the rate limiting energy barriers of 0.82 eV for the O + 2H reaction and 0.54 eV for the OH + OH reaction on the clean surface. On the oxygen modified Pt(100) surface, only the successive hydrogenation reaction pathway is located, with the rate-limiting energy barrier of 0.63 eV. In addition, it is clear that hydrogen prefers to combine with oxygen to form water rather than reacting with nitrogen to form ammonia. These results show that hydrogen is an efficient reductant on Pt based surfaces. 3.4. Insights from Microkinetic Modeling. Microkinetic modeling is employed to understand surface coverages, reaction rates, and product selectivity by combining the thermodynamic and kinetic information derived from DFT calculations.42−45 In this study, based on the Langmuir−Hinshelwood mechanism and the DFT calculated results of NO reduction by H2, we built a 11-step microkinetic model on the Pt(100) surface and a 13step model on the Md-Pt(100) surface. The adsorption and desorption of NO and H2 were assumed to be in equilibrium. The pseudo-steady-state approximation for other steps was applied. The reverse reactions of the elementary steps were not included on the clean Pt(100) surface since all the steps are nearly irreversible and the products are easy to desorb, while the reverse reaction in step 9 (NH2 + H ↔ NH3) and the desorption steps of NH3 and H2O on the Md-Pt(100) surface were considered. The entropy of the adsorption process was also considered and calculated from the vibrational frequencies of the respective states. The pre-exponential factor was calculated from the entropy differences between the initial and transition states of each elementary step.42,43 The detailed description of the microkinetic model is given in the Supporting Information. Note that the infinite separation energy barriers were used in the rate constant calculations. The microkinetic calculations were carried out under experimental reaction conditions, with pressures of 5 × 10−6 mbar (3.74 × 10−6 Torr)27 and 6 × 10−2 Torr,25 respectively, and temperatures ranging from 300 to 700 K. The energy barriers and the rate constants of the elementary steps on both surfaces at T = 500 K are tabulated in Tables 2 and 3, respectively. The product selectivity on both surfaces, Si = nri/

(2rN2 + 2rN2O + rNH3), where i represents the N-containing product and n is the number of nitrogen atoms in each Ncontaining molecule, is shown in Figure 7. It is seen from Figure 7a that, at a low pressure of 5 × 10−6 mbar and the ratio of NO/H2 = 1, the N2O selectivity is nearly 100% on the clean Pt(100) surface with temperatures ranging from 300 to 500 K, followed by rapid decrease as temperature increases; on the contrary, the selectivity toward N2 increases rapidly above 500 K and is larger than 80% at around 650 K. The NH3 formation is negligible in the whole temperature range. The results are consistent with the experimental observation on Pt catalysts that N2O is the main product at low temperatures (less than around 450 K).19,20 For the two pathways steps 10 and 11, the rate ratio of r11/r10 larger than 102 indicates that step 11, the disproportionation reaction OH + OH → H2O + O, is more favorable than the successive hydrogenation reaction for the H2O formation. Seen from Figure 7b, it is found that the MdPt(100) surface exhibits good product selectivity toward N2 (larger than 90%), while the N2O formation and NH3 formation are less competitive in the whole temperature range. Also, comparing the two elementary steps for the N2 formation, it is seen that although the rate constant for step 4 (N + N → N2) is larger than that for step 5 (NO + N → N2 + O), the latter reaction is more favorable below 550 K since the coverage of NO is much larger than the coverage of N at T = 300−550 K. Above 550 K, both steps 4 and 5 contribute to the N2 formation. At the NO/H2 ratio of 1:10, the microkinetic calculations show that the product selectivity on both surfaces has almost no change in the whole temperature range, due to the facts that hydrogen did not participate in the process of NO dissociation and NH3 formation is difficult on the two surfaces. Under the experimental condition of an intermediate pressure of 6 × 10−2 Torr for the subsurface oxygen formation, it is interesting to find that N2O is the unique product in the 300− 700 K temperature range on the clean surface, while the selectivity toward N-containing products is almost unchanged on Md-Pt(100), as shown in Figure 7. The NO conversion rate (%) is estimated by the expression of (2rN2 + 2rN2O + rNH3)/rNO. The present results show that the NO conversion rate is nearly 100% on both surfaces under the above experimental conditions, indicating that the dissociated NO molecule can be fully converted to N-containing products.

4. CONCLUSIONS We studied all the possible elementary steps of the H2-SCR of NO on clean and 0.50 ML subsurface oxygen-modified Pt(100) surfaces employing the first-principles DFT method combined with the microkinetic model. The results show that NO dissociation occurs via the direct bond scission pathway rather than via the H-assisted pathway on both surfaces, and the energy barriers for NO dissociation are close on both surfaces (0.86 eV on the clean Pt(100) surface vs 0.96 eV on the MdPt(100) surface). The N + N → N2 pathway is more favorable than the N-abstraction reaction pathway (NO + N → N2 + O) to form N2 from the point of view of the energy barrier on both surfaces, and the energy barriers for both pathways are reduced on the Md-Pt(100) surface compared to those on the clean Pt(100) surface. The energy barrier for N2O formation is also reduced on the Md-Pt(100) surface. In contrast, NH 3 formation is not competitive compared to other N-containing products on both surfaces. In addition, the produced surface oxygen can easily hydrogenate to form H2O on both surfaces,

Table 2. Calculated Energy Barriers E (eV) and Rate Constant k (s−1) on Clean Pt(100) at 500 K surface reaction step no.

reaction

rate equation

Eforward

1 2 3 4 4 6 7 8

NO(g) + ∗ ↔ NO* H2(g) + ∗ ↔ 2H* NO* + ∗ → N* + O* N* + N* → N2(g) + 2∗ N* + NO* → N2O(g) + 2* N* + H* → NH* + ∗ NH* + H* → NH2* + ∗ NH2* + H* → NH3(g) + 2∗

k3θNOθ* k4θN2 k5θNOθN k6θNθH k7θNHθH k8θNH2θH

0.86 0.67 0.84 0.73 0.81 1.35

2.10 7.26 5.18 3.82 5.08 3.75

9 10 11

O* + H* → OH* + ∗ OH* + H* → H2O(g) + 2∗ OH* + OH* → H2O(g) + O* + ∗

k9θOθH k10θOHθH k11θOH2

0.40 0.82 0.54

3.80 × 108 5.55 × 104 7.99 × 106

kforward

× × × × × ×

103 106 104 105 104 10−1

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DOI: 10.1021/acs.jpcc.5b07207 J. Phys. Chem. C 2015, 119, 24819−24826

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The Journal of Physical Chemistry C Table 3. Calculated Energy Barriers E (eV) and Rate Constant k (s−1) on Md-Pt(100) at 500 K surface reaction step no.

reaction

rate equation

Eforward

kforward

1 2 3 4 5 6 7 8 9

NO(g) + ∗ ↔ NO* H2(g) + ∗ ↔ 2H* NO* + ∗ → N* + O* N* + N* → N2(g) + 2∗ NO* + N* → N2(g) + O* + ∗ NO* + N* → N2O(g) + 2∗ N* + H* → NH* + ∗ NH* + H* → NH2* + ∗ NH2* + H* ↔ NH3* + ∗

k3θNOθ* k4θN2 k5θNOθN k6θNOθN k7θNθH k8θNHθH k9fθNH2θH − k9rθNH3θ*

0.96 0.50 0.58 0.76 0.77 0.76 0.98

1.30 1.57 1.62 2.50 8.14 4.28 2.48

10 11 12

O* + H* → OH* + ∗ OH* + H* → H2O* + ∗ NH3* → NH3(g) + ∗

k10θOθH k11θOHθH k12θNH3

0.63 0.60 1.20

1.20 × 106 2.12 × 107 2.38 × 10−5

13

H2O* → H2O(g) + ∗

k13θH2O

0.72

1.88

× × × × × × ×

102 107 106 104 104 104 103

Figure 7. Relative selectivity for the formation of N2, N2O, and NH3 on clean Pt(100) surface (a) and Md-Pt(100) surface (b) at a ratio of NO/H2 = 1 at different temperatures under experimental conditions (P = 5 × 10−6 mbar in ref 27 and P = 6 × 10−2 Torr in ref 25).

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indicating that the reductant H2 can effectively prevent the catalytic poisoning. The microkinetic modeling calculations confirm that N2O selectivity is almost 100% on the clean Pt(100) surface at T = 300−500 K and N2 selectivity is increased above 500 K at low pressure, while N2O selectivity is almost 100% during the whole temperature range at higher pressure. On the Md-Pt(100) surface, the selectivity toward N2 is larger than 90% during the whole temperature range at both low and intermediate pressures. The NH3 selectivity is almost negligible on both surfaces. The present study shows that the Pt(100) surface modified with subsurface oxygen has similar catalytic activity for NO dissociation and can enhance the N2 selectivity at low temperature. We expect the present study would provide valuable insights for improving the selectivity toward N2 of NO reduction on Pt-based catalysts.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 86-431-88498026. Tel.: 86-431-88498016. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grants 21373098 and 20973077) and the Program for New Century Excellent Talents in University (NCET). The authors are grateful to the reviewers for their valuable comments, which have significantly improved the manuscript.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07207. Detailed information on microkinetic modeling (PDF) 24825

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