NO Reduction by H2 on the Rh(111) and Rh(221) Surfaces: A

Article pubs.acs.org/JPCC

NO Reduction by H2 on the Rh(111) and Rh(221) Surfaces: A Mechanistic and Kinetic Study Li-yuan Huai, Tan Su, Hong Wen, Xin Jin, and Jing-yao Liu* Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: Periodic density functional theory (DFT) was used to investigate the selective catalytic reduction of NO by H2 (H2 SCR) on Rh(111) and stepped Rh(221) surfaces. The stepped Rh(221) surface exhibits a higher reactivity for NO reduction than the Rh(111) surface. NO dissociation on the Rh(221) surface exhibits almost no effect in the presence of H2, whereas predosed H atoms slightly inhibit NO dissociation on Rh(111). Microkinetic calculations further predicted the product selectivity for H2 SCR at different temperatures and pressures. It was found that, under ultrahigh-vacuum (UHV) conditions, NH3 is the only Ncontaining product on Rh(111), consistent with the experimental observations, whereas on the Rh(221) surface, N2O formation is predominant at low temperatures, and N2 becomes main product above 480 K. Under near-atmospheric-pressure conditions, the product selectivity on the Rh(111) surface exhibits almost no change, whereas N2O is the dominant product on Rh(221) throughout the whole temperature range. The present study indicates that the NO dissociation activity and product selectivity are strongly dependent on both the Rh surface structure and the experimental conditions.

1. INTRODUCTION Nitrogen oxide (NO) emitted from automobile exhaust and stationary sources is considered to be a primary atmospheric pollutant because of its well-known adverse effects on the environment and human health. The catalytic reduction of NO has attracted much attention and been studied extensively.1 Three-way catalysts (TWCs) containing supported Pt, Pd, and Rh metals have been successfully applied for the conversion of major pollutants from automotive exhaust to less harmful products.2−4 However, under oxygen-rich conditions, conventional TWCs are inefficient for NO reduction. Selective catalytic reduction (SCR), which has been developed over the past two decades, is one of the most promising approaches for reducing NO emissions from lean-burn and diesel engines. The SCR of NO by H2 as the reducing agent (H2 SCR) has received increasing attention recently because of its low operating temperature; on-board availability; and more importantly, production of H2O without any pollution.5−8 Among the noble metals of TWCs, Rh plays a dominant role in NO reduction because of its high catalytic reactivity toward NO decomposition. Intensive experimental and theoretical studies on NO adsorption and dissociation have been carried out on low-index Rh single-crystal surfaces [Rh(100), Rh(110), Rh(111)]9−13 and stepped surfaces [Rh(211), Rh(221), Rh(311), Rh(321), Rh(410), Rh(511), Rh(533)],4,14−17 with and without H2. The results showed that the surface activity of Rh is dependent on different surface structures. The presence of steps leads to lower activation barriers and increased exothermicity and, thus, higher surface reactivity. The kinetics of H2 SCR of NO has been studied mainly on low-index surfaces, especially on the Rh(111) surface,18−20 using lowenergy electron diffraction (LEED), simulated temperatureprogrammed desorption (TPD), mass spectrometry, and auger © 2016 American Chemical Society

electron spectroscopy. However, to the best of our knowledge, only a few experimental investigations have been done on the reaction of NO + H2 on a stepped surface of Rh.16,19 In this work, we focus on NO decomposition and reduction with H2 on flat Rh(111) and stepped Rh(221) surfaces. Illas and co-workers12,14 studied NO dissociation on these two surfaces by periodic density functional theory (DFT) and found that NO dissociation on the stepped (221) surface is thermodynamically and kinetically more favorable than that on the (111) surface. Several theoretical studies on the NO + H2 reaction have been performed on a few metal surfaces.21−25 The calculated results of reaction pathways for NO reduction by H2 on the Pt(111) surface suggested that the reaction proceeds through the addition of at least two H atoms to adsorbed NO, followed by cleavage of the N−O bond.21 The role of predosed atomic hydrogen in the NO dissociation was examined on the stepped Au(321) surface,22,23 and the results showed that NO dissociation is kinetically infeasible on the clean surface although it readily occurs by the pathway through a NOH intermediate on the H-predosed Au(321) surface. Our previous studies revealed that NO prefers to dissociate with the assistance of atomic H through the direct abstraction reaction on the Pd(111) surface,24 whereas NO dissociation occurs by the direct bond scission pathway rather than by the H-assisted pathway on the Pt(100) surface.25 In view of the different effects of H2 in the process of the SCR of NO, in the present study, we performed detailed first-principles calculations combined with microkinetic modeling of NO reduction by H2 on the Rh(111) and Rh(221) surfaces, aiming to elucidate Received: November 3, 2015 Revised: February 21, 2016 Published: February 24, 2016 5410

DOI: 10.1021/acs.jpcc.5b10740 J. Phys. Chem. C 2016, 120, 5410−5419

Article

The Journal of Physical Chemistry C

the species on the Rh(111) and Rh(221) surfaces are compiled in Table 1.

the mechanism and kinetics of NO reduction on both surfaces, determine the effects of H2 on this process, and predict the rate constants for elementary steps and the product selectivity under typical experimental conditions.

2. COMPUTATIONAL DETAILS All spin-polarized DFT calculations were carried out with the Vienna ab initio simulation package (VASP).26,27 Projectoraugmented-wave (PAW) pseudopotentials were utilized to describe the electron−ion interactions.28,29 The Perdew and Wang (PW91) functional30 with the generalized gradient approximation (GGA) was used to describe the exchangecorrelation effects. The energy cutoff for the plane-wave basis sets was set at 400 eV. The Brillouin zones were sampled with Monkhorst−Pack k-points31 of 15 × 15 × 15 for bulk calculations. Our calculated equilibrium lattice constant of 3.84 Å is in good agreement with the previous theoretical (3.83 Å)15 and experimental (3.80 Å)32 results. The Rh(111) surface was modeled by a (2 × 2) supercell with five periodic atom layers, and a vacuum thickness of 15 Å was used to remove any interactions between slabs. The bottom three Rh layers were fixed, and the rest were relaxed during geometry optimization. The Rh(221) surface was constructed in a (3 × 1) supercell with 10 atom layers and a vacuum thickness of 15 Å. The bottom five Rh layers were fixed, and the rest were relaxed. On both surfaces, a dipolemoment correction along the z direction was applied,33 and the calculations were stopped when the forces were less than 0.02 eV/Å. The Brillouin zone sampling was carried out using Monkhorst−Pack k-points of 7 × 7 × 1. The transition states (TSs) were well located using the climbing-image nudgedelastic-band (CI-NEB) method34,35 with eight intermediate images and verified by the existence of a single imaginary frequency based the vibrational analysis. All possible reaction pathways for each reaction were considered, but only the reaction paths with the lowest energy barriers are reported. Zero-point energy (ZPE) corrections were considered during all calculations. The adsorption energies (Ead) for the adsorbates were calculated as Ead = Eadsorbate − surface − Esurface − Eadsorbate (1)

Figure 1. Top and side views of the Rh(221) surface. TSE, BSE, HSE, and FSE refer to top, bridge, hcp, and fcc sites, respectively, on the step edge (SE). TT, BT, HT, and FT refer to top, bridge, hcp, and fcc sites, respectively, on the (111) terrace.

3.1. NO Dissociation with and without H2 on the Rh(111) and Rh(221) Surfaces. 3.1.1. Elementary Pathways on the Rh(111) Surface. NO adsorption was found to be most stable at the hcp hollow site through the N atom binding to the surface. The adsorption energy was found to be −2.47 eV, in good agreement with the previous theoretical result of −2.44 eV.15 Atomic N and O were found to prefer the hcp and fcc sites with adsorption energies of −5.35 and −5.31 eV, respectively. From the energy profiles depicted in Figure 2, the most favorable pathway for NO dissociation on the Rh(111) surface is from the initial state of NO at the hcp hollow site to the final state of coadsorbed N and O atoms at the adjacent fcc hollow sites. This process needs to overcome an energy barrier of 1.50 eV and is exothermic by 0.75 eV. The calculated energy barrier agrees well with the previous theoretical result of 1.53 eV.12 H2 adsorbs at the top site, parallel to the surface, with a weak adsorption energy of −0.46 eV. The bond length of H2 is increased from 0.75 Å in the gas phase to 0.97 Å. Atomic hydrogen binds preferentially at the fcc hollow site with an adsorption energy of −2.67 eV. The dissociation of molecular H2 to atomic H is an energy-barrierless process and is exothermic by 0.61 eV. To understand the role of H2 in NO dissociation, the following five possible pathways are all considered on the Rh(111) and Rh(221) surfaces; they are shown in Figures 3 and 4 for the Rh(111) and Rh(221) surfaces, respectively.

where Eadsorbate−surface, Esurface, and Eadsorbate represent the energies of the adsorbate on a surface [Rh(111) or Rh(221)], the corresponding surface, and the free adsorbate in the gas phase, respectively. The interaction energy was included and defined as the energy difference between the coadsorbed configuration (A*···B*) and the infinite-separation state (A* + B*) (i.e., the state with A and B at infinite distance is considered as the starting point). Unless otherwise specified, the reaction energy barrier of each elementary step was obtained as the energy difference between the transition state and the reactants at infinite separation (or one reactant at its most stable adsorption site), and the reaction energy was calculated as the energy difference between the product(s) and the reactant(s) at their respective most stable sites/infinite-separation states.

Path 1 +H

NO ⎯→ ⎯ N+O+H

Path 2 +H

NO ⎯→ ⎯ N + OH

Path 3 +H

NO ⎯→ ⎯ NOH → N + OH

3. RESULTS AND DISCUSSION There are four possible adsorption sites (top, bridge, hcp hollow, and fcc hollow) on the Rh(111) surface, and 16 possible adsorption sites on the Rh(221) surface. The top and side views for the stepped Rh(221) surface and the relative adsorption sites are presented in Figure 1. The adsorption sites and adsorption energies for the most stable configurations of

Path 4 +H

NO ⎯→ ⎯ NH + O

Path 5 +H

NO ⎯→ ⎯ HNO → NH + O 5411


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The Journal of Physical Chemistry C Table 1. Geometric and Energy Parameters of Reaction Intermediates as Identified in the Stable State Rh(111) reaction intermediate

configuration

bond length(s) (Å)

Rh(221) adsorption energy, Eads(eV) −2.47 −0.46 −2.67 −5.35 −5.31 −3.35 −2.28

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

N2O

hcp, through N top, through H fcc hcp fcc hcp, through N hcp, through N and O top, through N

d(N−Rh) = 1.99

−0.33

N2 NH NH2 NH3 OH H2O

top, through N hcp, through N bridge, through N top, through N fcc, through O top, through O

d(N−Rh) d(N−Rh) d(N−Rh) d(N−Rh) d(O−Rh) d(O−Rh)

−0.52 −4.44 −2.70 −0.68 −2.91 −0.34

NO H2 H N O NOH HNO

= = = = = = =

= = = = = =

2.05 1.66 1.86 1.92 2.00 1.99 2.07, d(O−Rh) = 2.07

1.93 1.99 2.08 2.15 2.13 2.34

configuration BSE, through N TSE, through H BSE BSE BSE BSE, through N FT‑1, through N and O BSE, through two Ns TSE, through N BSE, through N BSE, through N TSE, through N BSE, through O TSE, through O

bond length(s) (Å)

adsorption energy, Eads(eV)

1.95 1.71 1.76 1.82 1.90 1.92 2.06, d(O−Rh) = 2.03

−2.75 −0.60 −2.63 −5.25 −5.46 −3.32 −2.43

d(N−Rh) = 1.94 d(N−Rh) = 2.04

−0.76

d(N−Rh) d(N−Rh) d(N−Rh) d(N−Rh) d(O−Rh) d(O−Rh)

−0.80 −4.26 −3.21 −0.91 −3.38 −0.51

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

= = = = = = =

= = = = = =

1.94 1.91 2.07 2.16 2.09 2.27

Figure 2. Pathways for NO dissociation on the Rh(111) and Rh(221) surfaces.

formation of the HNO intermediate (path 5) from coadsorption configuration II, NOhcp···Hhcp. The energy barriers for these three elementary steps (NO + H → NH + O, NO + H → HNO → NH + O) are 1.58, 1.58, and 0.75 eV, respectively, and the reaction energies are −0.84, 0.77, and −1.61 eV, respectively. 3.1.2. Elementary Pathways on the Rh(221) Surface. The NO, N, and O species prefer to adsorb at the BSE site with adsorption energies of −2.75, −5.25 and −5.46 eV, respectively. NO dissociation starts from the most favorable configuration at the BSE site and finishes with the N atom staying at the BSE site and the O atom at the BT‑4 site. The energy barrier is 1.25 eV with a reaction exothermicity of 0.60 eV, indicating that NO decomposition is more likely to occur on the stepped (221) surface than on the (111) surface. The calculated results for both the adsorption energy and dissociation barrier of NO are in agreement with the previous theoretical results of −2.70 and 1.17 eV,14 respectively. H2 was found to favor adsorption at the TSE site, with the molecular axis parallel to the surface, whereas atomic H prefers to adsorb on the BSE site. The adsorption energies for H2 and H are −0.60 and −2.63 eV, respectively. The dissociation of H2 is also a barrierless process with a reaction energy of −0.40 eV. The NOH fragment is preferentially adsorbed on the BSE site through the N atom, and HNO prefers the FT‑1 site through N

Our calculated results show that both NOH and HNO species prefer to adsorb at the hcp hollow site with adsorption energies of −3.35 and −2.28 eV, respectively. NOH adsorbs through the N-terminus with the N−O bond perpendicular to the surface and a N−O−H angle of 104.72°. The intermediate HNO binds to the surface in an η1(O)−η2(N) mode in which the H−N−O angle is 109.42°. As can be seen in Figure 3, the initial state of structure I, NOhcp···Hfcc, has an attractive energy of 0.03 eV and undergoes the dissociation of NO. During this process, H acts as a spectator. The reaction energy barrier is increased from 1.50 eV on the clean surface to 1.79 eV on the H-predosed surface with a reaction energy of −0.83 eV. Starting from coadsorption configuration II, NOhcp···Hhcp, with a small repulsive energy of 0.13 eV, the coadsorbed product Nhcp···OHbridge is obtained either through the direct abstraction reaction (path 2) or through the formation of the NOH intermediate (path 3). Path 2 has an energy barrier of 1.60 eV and is exothermic by 0.27 eV. In path 3, the energy barrier is 1.55 eV for the formation of NOH, and the dissociation process to form the coadsorbed products is barrierless; the reaction energies for these two elementary steps are 1.28 and −1.55 eV, respectively. Similarly, NO dissociation assisted by predosed H atoms to form NHfcc··· Ofcc occurs by the direct abstraction reaction (path 4) from coadsorption configuration III, NOtop···Hhcp, or by the 5412


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Figure 3. Pathways for NO dissociation assisted by H2 on the Rh(111) surface. Figure 4. Pathways for NO dissociation assisted by H2 on the Rh(221) surface.

and O with adsorption energies of −3.32 and −2.43 eV, respectively. NO dissociation according to path 1 starts from coadsorbed configuration I′, NOBSE···HBSE, to form the final state NBSE···OBT‑4···HBSE. This process needs to overcome an energy barrier of 1.25 eV and has a reaction energy of −0.59 eV. Compared with NO direct dissociation, the coadsorbed H atom has no effect on NO dissociation by path 1 on the Rh(221) surface. Path 2 starts from coadsorbed configuration II′, NOBSE···HHT‑4, and finishes with the coadsorbed product NBSE···OHBT‑4. The energy barrier for this direct abstraction path is 1.47 eV, and the reaction is exothermic by 0.58 eV. The initial state of path 3 is NOBSE···HBT‑4 (III′), and the final state is NBSE···OHBT‑4. The energy barriers for the two steps involved in path 3 are 1.03 and 0.35 eV, and the reaction energies are 1.01 and −1.59 eV, respectively. For the dissociation product of NH and O, path 4 starts from coadsorbed configuration V′, NOBSE··· HFT‑4, to form NHHSE···OHT‑1 (VI′) by the direct abstraction reaction, and path 5 starts from coadsorbed configuration IV′, NOHT‑1···HHT‑1, to form NHFT‑1···OFT‑1 (VII′) through the HNO intermediate. The energy barriers for these three steps (NO + H → NH + O, NO + H → HNO → NH + O) are 1.76, 1.89, and 0.72 eV, respectively, and the reaction energies are −0.55, 1.16, and −1.71 eV, respectively. 3.2. Formation Mechanisms for Products on the Rh(111) and Rh(221) Surfaces. 3.2.1. Formation Mechanism for N2. On the Rh(111) surface, N2 is weakly bound to the top site with the N2 molecule perpendicular to the surface. The adsorption energy is −0.52 eV. There are two possible pathways for N2 formation. As shown in Figure 5, starting from

the initial state of Nhcp···Nhcp, the combination reaction (N + N → N2) occurs through TS9 by overcoming a higher energy barrier of 2.40 eV. The formed N2 readily desorbs from the surface because of the adsorption energy of only −0.06 eV on the bridge site. The direct abstraction reaction N + NO → N2 + O, which starts from initial state of Nfcc···NOfcc (IV) and proceeds to the final state of N2top···Ofcc, needs to overcome an energy barrier of 2.28 eV and is exothermic by 0.89 eV. On the Rh(221) surface, N2 prefers the TBE site through one N atom with an adsorption energy of −0.80 eV. The recombination pathway for N 2 formation starts from coadsorbed state of NBSE···NBT‑4 and finishes with N2 at the FT‑3 site, where two N atoms bind to the BSE and BT‑4 sites. The energy barrier for this reaction is 1.26 eV, and the formed N2 molecule at the FT‑3 site has a small adsorption energy of −0.12 eV and thus desorbs easily from the surface. For the abstraction reaction pathway, starting from coadsorption configuration NOBSE···NBT‑4, the reaction occurs through TS10′ to form N2TT‑2···OBSE. The energy barrier of this pathway is 1.75 eV, and the reaction energy is −1.11 eV. 3.2.2. Formation Mechanism for N2O. On the Rh(111) surface, the top adsorption of N2O is the most stable (−0.33 eV), with the whole molecule perpendicular to the surface. As shown in Figure 6, starting from coadsorption configuration NOhcp···Nhcp (V), the reaction must surmount an energy barrier of 1.90 eV, and the N2O formed at the bridge site is likely to desorb because of a very low adsorption energy (−0.11 eV). On the Rh(221) surface, N2O prefers to adsorb at the BSE site 5413


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Figure 5. Pathways for N2 formation on the Rh(111) and Rh(221) surfaces.

through two N atoms with an adsorption energy of −0.76 eV. Figure 6 shows that the reaction starts from the initial state of NOBSE···NBT‑4 and passes through transition state TS11′ by overcoming an energy barrier of 1.69 eV to form a less stable configuration of N2O, which binds to BSE and FT‑3 sites through the two N atoms with an adsorption energy of −0.55 eV. 3.2.3. Formation Mechanism for NH3. Figure 7 shows the reaction profiles of the successive hydrogenation steps on both surfaces. On the Rh(111) surface, the NH fragment prefers the hcp hollow site, NH2 is most stable on the bridge site with its C2 axis perpendicular to the surface, and NH3 adsorbs only on the top site. The adsorption energies for these three fragments are −4.44, −2.70, and −0.68 eV, respectively. The initial states for the hydrogenation reactions of NHx for x = 0−2 are Nhcp··· Hhcp, NHhcp···Hhcp, and NH2bridge···Hhcp, respectively, with repulsive energies of 0.25, 0.20, and 0.08 eV and final states of NHhcp, NH2bridge, and NH3top, respectively. The energy barriers for these three steps are 1.00, 0.94, and 1.09 eV, respectively. On the Rh(221) surface, the NH and NH2 fragments prefer to adsorb on the BSE site, and NH3 prefers the TSE site, with adsorption energies of −4.26, −3.21, and −0.91 eV, respectively. The successive hydrogenation step for NH3 formation starts from coadsorption configurations NBSE···HBT‑4, NBSE···HFT‑3, and NH2BSE···HBSE, respectively. Then, the reaction proceeds with energy barriers of 0.93, 0.92, and 1.17 eV to form NHBSE, NH2BSE, and NH3TSE, respectively. 3.2.4. Formation Mechanism for H2O. Surface oxygen atoms can be formed by NO reduction as discussed above. As shown in Figure 8, the hydrogenation reaction of an O atom

Figure 6. Pathways for N2O formation on the Rh(111) and Rh(221) surfaces.

5414


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Figure 7. Pathways for NH3 formation on the Rh(111) and Rh(221) surfaces.

Figure 8. Pathways for H2O formation on the Rh(111) and Rh(221) surfaces.

starts from the initial state Ofcc···Hfcc and ends with OH at the most favorable fcc site. This step needs to surmount an energy barrier of 0.64 eV and is endothermic by 0.56 eV. Starting from the OH intermediate, H2O can be formed by either the hydrogenation reaction or the disproportionation reaction. The initial states for the two pathways are OHfcc···Htop and OHbridge···OHbridge, respectively, and the final states are H2Otop and H2Otop···Ohcp, respectively. The energy barriers for the two pathways are 0.66 and 0.15 eV, respectively, and the reaction energies are −0.04 and −0.60 eV, respectively. On the Rh(221) surface, the most stable sites for the OH fragment and H2O are the BSE and TSE sites, respectively, with adsorption energies of −3.38 and −0.51 eV, respectively. The hydrogenation reactions start from coadsorption configurations

OBSE···HBT‑4 and OHBSE···HBSE to form OHBSE and H2OTSE, respectively. The energy barriers for these two steps are 0.89 and 1.00 eV, respectively, and the reaction energies are 0.02 and 0.42 eV, respectively. The disproportionation reaction starts from the initial state OHBSE···OHBT‑4 with a repulsive energy of 0.43 eV and passes through TS17′ to form the final state H2OTT‑2···OBSE. This process has an energy barrier of 0.44 eV and is endothermic by 0.41 eV. 3.3. Comparison of the Mechanism of NO Reduction by H2 between Rh(111) and Rh(221) Surfaces. As discussed above, in the absence of H2, the effective energy barriers for NO direct dissociation are 1.50 and 1.25 eV on the Rh(111) and Rh(221) surfaces, respectively, indicating that the 5415


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The Journal of Physical Chemistry C Table 2. Calculated Energy Barriers E (eV) and Rate Constants k (s−1) on the Clean Rh(111) Surface at 400 K step no.

reaction

rate equation

Eforward

kforward

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14

NO(g) + * ↔ NO* H2(g) + * ↔ 2H* NO* + * → N* + O* NO* + H* → N* + OH* NO* + H* ↔ NOH* + * NOH* + * → N* + OH* NO* + H* → NH* + O* NO* + H* ↔ HNO* + * HNO* + * → NH* + O* N* + N* → N2(g) + 2* NO* + N* → N2(g) + O* NO* + N* → N2O(g) + 2* N* + H* → NH* + * NH* + H* ↔ NH2* + *

k3θNOθ* k4θNOθH k5fθNOθH − k5rθNOHθ* k6θNOHθ* k7θNOθH k8fθNOθH − k8rθHNOθ* k9θHNOθ* k10θN2 k11θNOθN k12θNOθN k13θNθH k14fθNHθH − k14rθNH2θ*

1.50 1.60 1.55 0.00 1.58 1.58 0.75 2.40 2.28 1.90 1.00 0.94

1.16 1.79 1.28 8.72 5.95 6.05 9.46 3.38 2.62 2.51 9.71 5.85

R15

NH2* + H* → NH3(g) + 2*

k15θNH2θH

1.09

1.41 × 101

R16 R17

O* + H* ↔ OH* + * OH* + OH* → H2O(g) + O* + *

k16fθOθH − k16rθOHθ* k17θOH2

0.64 0.15

1.31 × 105 4.71 × 1011

× × × × × × × × × ×

10−6 10−7 10−6 1012 10−7 10−6 103 10−17 10−16 10−11

× 101

pseudo-steady-state approximation for other steps was applied, where the rates for the production and consumption of each species were assumed to be equal. All exothermic steps were assumed to be irreversible, and all endothermic steps were assumed to be reversible. The desorption steps of products on the Rh(111) surface were ignored because the products are easy to desorb, whereas the desorption step of H2O was included on the Rh(221) surface. The rate constant was calculated based on harmonic transition state theory (TST)39 according to the expression

Rh(221) surface exhibits better activity than the Rh(111) surface. The presence of H2 has no effect on N−O bond scission through the direct pathway (NO + H → N + O + H) on the Rh(221) surface with the same energy barrier as on clean surface, whereas it has a slightly inhibitory effect for NO dissociation on the Rh(111) surface. Also, the intermediates NOH and HNO are thermodynamically unstable on both surfaces. On the Rh(111) surface, the energy barriers for the two pathways are rather high, at 2.28 eV for the N-abstraction reaction (N + NO → N2 + O) and 2.40 eV for the combination reaction (N + N → N2). The formation of N2O is also difficult, with a barrier of 1.90 eV, whereas the overall energy barrier for NH3 formation is 1.26 eV. Accordingly, NH3 is mainly produced, whereas the formations of N2 and N2O are unfavorable. However, the situation becomes quite different on the Rh(221) surface. N2O formation is still unfavorable because of the higher energy barrier (1.69 eV), whereas the energy barrier for N2 formation through the combination reaction is significantly reduced to 1.26 eV, similar to the overall energy barrier for NH3 formation (1.17 eV). Thus, just from an energetic point of view, the product selectivity on the Rh(221) surface cannot be clearly clarified. Therefore, we established a microkinetic model36,37 to determine the reaction rates and product selectivities on both surfaces based on the density functional theory results. In addition, for the formation of H2O on both surfaces, we found that the disproportionation reaction of OH fragments is kinetically and thermodynamically more favorable than the hydrogenation reaction for H2O formation. 3.4. Microkinetic Model. To obtain quantitative estimates of the rates of the elementary steps and the selectivities of the main products of NO reduction by H2 under the reaction conditions, we developed a microkinetic model comprising 17 elementary steps on the Rh(111) surface and 14 elementary steps on the Rh(221) surface. The adsorption and desorption of NO and H2 were assumed to be in equilibrium, and the equilibrium constant Keq was estimated according to the expression Keq = exp[−(ΔEads − TΔS)/kBT], where ΔEads is the adsorption energy of NO or H2 and ΔS is the entropy change of NO or H2 induced by adsorption. The gas-phase entropy was obtained from NIST Chemistry WebBook.38 The

⎛ −E ⎞ k = vi exp⎜ a ⎟ ⎝ RT ⎠

(2)

where vi is the pre-exponential factor, Ea is the ZPE-corrected energy barrier, and T is the temperature. The pre-exponential factor vi of each reaction pathway was determined from DFTcalculated vibrational frequencies as follows 3N

vi =

∏i = 1 fiIS 3N − 1

∏i = 1 fiTS

(3)

where f IS i represents the vibrational frequencies at the initial state and f TS represents the vibrational frequencies at the i transition state (excluding the imaginary one). The site balance of the intermediate species involved in the reaction and the free site (*) can be written in terms of coverage (θX, where X represents a surface species). The coverages of surface NO and H were obtained as θNO = PNOK1θ* and θH = PH21/2K21/2θ*. The coverages for other surface species can be described according to the steady-state approximation, where the rates for the production and consumption of each species are equal. kf

Consider an elementary step A* + B* ↔ C* + D*, where * kr

denotes an adsorbed species. According to transition state theory, the rate of reaction for this elementary step can be given as r = rf − rr = k f θAθB − k rθCθD

(4)

where rf and rr denote the rates of the forward and reverse reactions, respectively, and θA, θB, θC, and θD denote the surface coverages of species A, B, C, and D, respectively. The rate 5416


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The Journal of Physical Chemistry C Table 3. Calculated Energy Barriers E (eV) and Rate Constants k (s−1) on Rh(221) at 400 K step no.

reaction

rate equation

Eforward

kforward

R1′ R2′ R3′ R4′ R5′ R6′ R7′ R8′ R9′ R10′

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

k3θNOθ* k4θNOθ* k5θNOθH k6θNOθN k7θNOθN k8fθNθH-k8rθNHθ* k9θNOθH k10θNH2θH

1.25 1.25 1.26 1.75 1.69 0.93 0.92 1.17

1.13 9.87 1.78 3.50 3.35 1.56 1.48 4.61

R11′ R12′ R13′ R14′

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

k11θOθH k12θOHθH k13θOH2 k14θH2O

0.89 1.00 0.44 0.51

2.20 × 102 4.49 1.05 × 108 1.19 × 105

× × × × × × × ×

10−3 10−4 10−3 10−10 10−9 102 102 10−2

Figure 9. Relative selectivities for the formation of N2, N2O, and NH3 on the (a) Rh(111) and (b) Rh(221) surfaces at a NO/H2 ratio of 1 at different temperatures under the experimental conditions.

when the temperature is above 480 K; on the contrary, the selectivity toward N2 increases rapidly and is greater than 80% in the temperature range of 480−600 K, which indicates that the Rh(221) surface exhibits higher N2 selectivity for the SCR of NO by H2 in this temperature range. For the two pathways of N2 formation, it was found that, although the rate constant for R5′ is larger than that for R6′, the much larger coverage of NO compared to N atoms makes R6′ the dominant N2 formation pathway below 450 K, whereas R5′ becomes the main N2 formation pathway at temperatures above 450 K. The large coverage of NO is also the reason that N2O is the main Ncontaining product at low temperature. Moreover, as shown in the figure, the selectivity toward NH3 is zero throughout the whole temperature range, even though the formation energy barrier for NH3 is slightly lower than that for N2. The kinetic results further demonstrate that, for some reactions, especially those with similar energy barriers, the product selectivity cannot be identified just based on the energetics, and DFT calculations coupled with microkinetic modeling is a viable way for the prediction of reaction kinetics. Under the near-atmospheric-pressure conditions (PNO = 4.1 kPa,PH2 = 45.7 kPa, and T = 300−600 K), the rate calculations show that the onset of the formation of all products shifts to higher temperature compared to that under UHV conditions (see Tables S5 and S6 in Supporting Information). Still, it was found that, for NH3 production, there is nearly 100% selectivity on Rh(111) but no selectivity on Rh(221) over the whole

equations, reaction energy barriers, and rate constants at 400 K for the elementary steps on both surfaces are compiled in Tables 2 and 3. The reaction rates of the elementary steps at different temperatures and pressures on both surfaces are reported in Tables S1−S4. The product selectivity is defined as the relative rate of each product, that is, Si = nri/(2rN2 + 2rN2O + rNH3), where i represents a N-containing product and n is the number of nitrogen atoms in each molecule of the Ncontaining product. The selectivities for N-containing products as functions of temperature on both surfaces are shown in Figure 9. Microkinetic modeling was carried out under both typical ultra-high-vacuum (UHV) conditions (P = 10−6 mbar and T = 300−600 K)18,19,40 and near-atmospheric-pressure conditions (PNO = 4.1 kPa, PH2 = 45.7 kPa, and T = 300−600 K).7 The results in Figure 9a show that, at a low pressure of 1 × 10−6 mbar and a NO/H2 ratio of 1, NH3 is the predominant product on the Rh(111) surface in the temperature range from 300 to 600 K, and the selectivities toward N2 and N2O are zero. This is in good agreement with the experimental result that a substantial amount of NH3 is produced on the Rh(111) surface.18 On the stepped Rh(221) surface, as shown in Figure 9b, different product selectivities were obtained. Specifically, the selectivity for N2O is greater than 80% below 450 K, which means that N2O is the predominant product and N2 and NH3 are less competitive. As the temperature is increased, the selectivity for N2O decreases dramatically and is less than 10% 5417


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the Research Project of the Science and Technology of Jilin Province, China (Grant 201413001). The authors are grateful to the referees for their valuable comments, which significantly improved the manuscript.

temperature range, exactly as found for UHV conditions. However, as shown in Figure 9b, the changes in the product selectivities toward N2O and N2 as a function of temperature are different under the two sets of experimental conditions on the Rh(221) surface. The selectivity toward N2O decreases gradually from nearly 95% at 300 K to about 85% at 600 K; meanwhile, the selectivity to N2 is about 5% at 300 K and reaches 15% at 600 K. This suggests that, under such experimental conditions, Rh(221) exhibits a high selectivity for the production of N2O but a low selectivity for N2 in the temperature regime from 300 to 600 K.

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(1) Liu, Z.; Ihl Woo, S. Recent Advances in Catalytic DeNOx Science and Technology. Catal. Rev.: Sci. Eng. 2006, 48, 43−89. (2) Ge, Q.; Neurock, M. Structure Dependence of NO Adsorption and Dissociation on Platinum Surfaces. J. Am. Chem. Soc. 2004, 126, 1551−1559. (3) Paredis, K.; Ono, L. K.; Behafarid, F.; Zhang, Z.; Yang, J. C.; Frenkel, A. I.; Cuenya, B. R. Evolution of the Structure and Chemical State of Pd Nanoparticles During the in Situ Catalytic Reduction of NO with H2. J. Am. Chem. Soc. 2011, 133, 13455−13464. (4) Loffreda, D.; Simon, D.; Sautet, P. Structure Sensitivity for NO Dissociation on Palladium and Rhodium Surfaces. J. Catal. 2003, 213, 211−225. (5) Liu, Z.; Li, J.; Woo, S. I. Recent Advances in the Selective Catalytic Reduction of NOx by Hydrogen in the Presence of Oxygen. Energy Environ. Sci. 2012, 5, 8799−8814. (6) Hamada, H.; Haneda, M. A Review of Selective Catalytic Reduction of Nitrogen Oxides with Hydrogen and Carbon Monoxide. Appl. Catal., A 2012, 421−422, 1−13. (7) Stenger, H. G.; Hepburn, J. S. Nitric Oxide Reduction by Alumina-Supported Rhodium, Palladium, and Platinum. 1. Intrinsic Activities and Selectivities. Energy Fuels 1987, 1, 412−416. (8) Kobylinski, T. P.; Taylor, B. W. The Catalytic Chemistry of Nitric Oxide: II. Reduction of Nitric Oxide over Noble Metal Catalysts. J. Catal. 1974, 33, 376−384. (9) Mavrikakis, M.; Rempel, J.; Greeley, J.; Hansen, L. B.; Nørskov, J. K. Atomic and Molecular Adsorption on Rh(111). J. Chem. Phys. 2002, 117, 6737−6744. (10) Zeng, Z. H.; Da Silva, J. L. F.; Li, W. X. Theory of Nitride Oxide Adsorption on Transition Metal (111) Surfaces: A First-Principles Investigation. Phys. Chem. Chem. Phys. 2010, 12, 2459−2470. (11) Inderwildi, O. R.; Lebiedz, D.; Deutschmann, O.; Warnatz, J. Influence of Initial Oxygen Coverage and Magnetic Moment on the NO Decomposition on Rhodium (111). J. Chem. Phys. 2005, 122, 154702. (12) González, S.; Sousa, C.; Illas, F. Promoter and Poisoning Effects on NO-Catalyzed Dissociation on Bimetallic RhCu(111) Surfaces. J. Catal. 2006, 239, 431−440. (13) Tian, K.; Tu, X. Y.; Dai, S. S. NO Dissociation Pathways on Rh(100), (110), and (111) Surfaces: A Comparative Density Functional Theory Study. Surf. Sci. 2007, 601, 3186−3195. (14) González, S.; Loffreda, D.; Sautet, P.; Illas, F. Theoretical Study of NO Dissociation on Stepped Rh(221) and RhCu(221) Surfaces. J. Phys. Chem. C 2007, 111, 11376−11383. (15) Rempel, J.; Greeley, J.; Hansen, L. B.; Nielsen, O. H.; Nørskov, J. K.; Mavrikakis, M. Step Effects on the Dissociation of NO on ClosePacked Rhodium Surfaces. J. Phys. Chem. C 2009, 113, 20623−20631. (16) Wolf, R. M.; Bakker, J. W.; Nieuwenhuys, B. E. Dissociation of Nitric Oxide and Reaction with Hydrogen on Rh(111) and Various Stepped Rh(111) Surfaces. Surf. Sci. 1991, 246, 135−140. (17) Janssen, N. M. H.; Cholach, A. R.; Ikai, M.; Tanaka, K.; Nieuwenhuys, B. E. The Interaction of NO with Stepped Rh Surfaces. Surf. Sci. 1997, 382, 201−213. (18) Schaak, A.; Imbihl, R. Spiral Waves and Formation of Low Work Function Areas in Catalytic NO Reduction with Hydrogen on a Rh(111) Surface. J. Chem. Phys. 2002, 116, 9021−9027. (19) Makeev, A. G.; Slinko, M. M.; Janssen, N. M. H.; Cobden, P. D.; Nieuwenhuys, B. E. Kinetic Oscillations and Hysteresis Phenomena in the NO + H2 Reaction on Rh(111) and Rh(533): Experiments and Mathematical Modeling. J. Chem. Phys. 1996, 105, 7210−7222. (20) Cobden, P. D.; Janssen, N. M. H.; van Breugel, Y.; Nieuwenhuys, B. E. Non-Linear Behaviour in the NO + H2 Reaction over Rh(111). Surf. Sci. 1996, 366, 432−444.

4. CONCLUSIONS The adsorption of NO and its reduction by H2 on the Rh(111) and Rh(221) surfaces have been studied using periodic DFT calculations. Our results show that the Rh(221) surface exhibits better activity toward NO dissociation than the Rh(111) surface. The presence of H2 has a slightly inhibitory effect for NO dissociation on the Rh(111) surface but no effect on NO dissociation on Rh(221) surface, with an energy barrier of 1.25 eV. For the formation of N-containing products, the formations of both N2 and N2O are kinetically unfavorable, and NH3 is the main product on the Rh(111) surface. On the Rh(221) surface, the energy barrier for N2 formation is significantly reduced through the recombination reaction of N atoms, and the energy barrier for NH3 formation is slightly increased, whereas N2O formation is still infeasible. The results of microkinetic analysis further indicate that, under UHV conditions (P = 10−6 mbar and T = 300−600 K), NH3 is the most favorable product on the Rh(111) surface whereas N2O is the dominant product on the Rh(221) surface at low temperatures and the N2 selectivity becomes greater than 80% above 480 K. Under nearatmospheric-pressure conditions (PNO = 4.1 kPa, PH2 = 45.7 kPa), the product selectivity on the Rh(111) surface undergoes almost no change, whereas the Rh(221) surface exhibits a high selectivity for the production of N2O throughout the whole temperature range considered.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10740. Microkinetic model, calculated reaction rates for elementary steps on the Rh(111) and Rh(221) surfaces under typical experimental conditions (P = 10−6 mbar and T = 300−600 K) and near-atmospheric-pressure conditions (PNO = 4.1 kPa, PH2 = 45.7 kPa, and T = 300− 600 K), and calculated reaction rates for N-containing products on the Rh(111) and Rh(221) surfaces under both sets of experimental conditions (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grants 21373098 and 21501062) and 5418


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NO Reduction by H2 on the Rh(111) and Rh(221) Surfaces: A

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