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Nitric Oxide Adsorption and Reduction Reaction Mechanism on the Rh7+ Cluster: A Density Functional Theory Study Hujun Xie,*,† Meng Ren,† Qunfang Lei,‡ and Wenjun Fang‡ † ‡
Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China Department of Chemistry, Zhejiang University, Hangzhou 310027, China
bS Supporting Information ABSTRACT: The transition metal rhodium has been proved the effective catalyst to convert from NOx to N2. In the present work, we are mainly focused on the NO adsorption and decomposition reaction mechanism on the surface of the Rh7+ cluster, and the calculated results suggest that the reaction can proceed via three steps. First, the NO can adsorb on the surface of the Rh7+ cluster; second, the NO decomposes to N and O atoms; finally, the N atom reacts with the second adsorbed NO and reduces to a N2 molecule. The N O bond breaks to yield N and O atoms in the second step, which is the rate-limiting step of the whole catalytic cycle. This step goes over a relatively high barrier (TS12) of 39.6 kcal/mol and is strongly driven by a large exothermicity of 55.1 kcal/mol during the formation of stable compound 3, accompanied by the N and O atoms dispersed on the different Rh atoms of the Rh7+ cluster. In addition, the last step is very complex due to the different possibilities of reaction mechanism. On the basis of the calculations, in contrast to the reaction path II that generates N2 from two nitrogen atoms coupling, the reaction path I for the formation of intermediate N2O is found to be energetically more favorable. Present work would provide some valuable fundamental insights into the behavior of the nitric oxide adsorption and reduction reaction mechanism on the Rh7+ cluster.
1. INTRODUCTION Metallic rhodium has proven to be one of the best catalysts for the conversion of NOx to molecular nitrogen, a very important step in pollution-control processes.1,2 It has been thought that the key property of any viable catalyst for NOx reduction is its ability to break the N O bond of the reactant.3 6 As a result of its practical importance, the reduction of nitric oxide at transition metal surfaces has been widely investigated due to its importance in environmental pollution control processes.7 11 The remarkable activity of rhodium clusters and surfaces for adsorption and reduction of NO has received particular attentions from the surface science areas.12 19 The relatively low N O bond energy may lead to a great degree of dissociation for surface adsorbed nitric oxide. Loffreda et al.20 have investigated the chemisorption states of NO on Rh(100) and (111) surfaces in which two different coverages of NO for each of the surfaces were taken into account by density functional theory (DFT) calculations in connection with periodic boundary conditions (PBC). It was found that the NO molecules can adsorb on Rh(100) and (111) surfaces via various sites. It was also revealed that the absolute values of the binding energies of NO molecules with the metal surfaces decreased with the increment of the coverages of NO. Mannstadt et al.21 have calculated the chemisorption states of NO molecules on Rh(100) surface using the full potential linearized augmented-plane-wave (FLAPW) r 2011 American Chemical Society
method. It is found that the values of the vibrational frequency of the N O stretching mode (υNO) on the Rh surface correspond to 1634 cm 1. In contrast to the inherent vibrational frequency of the N O bond (1876 cm 1), a present result suggested that a weaker N O bond was formed on the Rh(100) surface. In a recent paper, Torres and co-workers22 carried out DFT calculations to investigate the adsorption of NO on the Rh6+ cluster. The results showed the dissociative adsorption as the most stable configuration in both the octahedron and the prism Rh6+ clusters. Burch et al.23 performed DFT calculations via PBC on the adsorption states of Rh atoms on a MgO(001) surface as well as the adsorption of NO on these models. They found that the supported Rh atom can form a strong bond with the NO molecule. Ghosh et al.24 have studied the binding of NO to small Rh clusters including one to five atoms via DFT calculations. It was shown that the N O bond in Rh clusters are stronger than that in Rh(100) or Rh(111) surfaces, suggesting that Rh clusters may be good catalysts for NO reduction. This local effect results in reducing the magnitude of the NO binding energy. Received: May 12, 2011 Revised: October 26, 2011 Published: October 26, 2011 14203
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The Journal of Physical Chemistry A Previous researches showed that the nitric oxide (NO) can be decomposed on the rhodium clusters Rhn+ (6 < n < 30) by means of Fourier transfer ion cyclotron resonance (FT-ICR) mass spectrometry.25 29 The experiments involving prolonged exposure to NO have revealed interesting size-dependent trends in the mechanism and efficiency of multiple NO molecule decomposition via the stepwise reaction; the mechanism is thought to proceed via the dissociative adsorption of NO on the cluster surface. Borg et al.30 have reported that the dissociation of NO on the Rh(111) surface is hindered at high coverage, indicating that several empty, adjacent, three-coordinated hollow sites are necessary for the dissociation of NO under these conditions. Zaera et al.31 34 have carried out experimental studies on the deposition of atomic nitrogen on the Rh(111) surface by means of effusive collimated beams. Isotopic labeling experiments were performed where 14N-dosed surfaces were subsequently exposed to 15NO. Subsequent temperature-programmed desorption suggested the distribution of isotopes in the resulting molecular nitrogen, that is, the yield of the mixed 14N15N isotopomer. Monte Carlo simulations were used to explain the observed isotopic distributions in terms of the formation of islands with the nitrogen isotopes distributed in a layered structure, with the 14N atoms in a core surrounded by a 15N outer shell.34 Zaera et al.31 also found the evidence for an N2O intermediate in the catalytic reduction of NO to N2 on Rh(111) surfaces by using a molecular beam technique. It was shown that the two nitrogens of N2 come from different sources; this result provides direct kinetic evidence for a mechanism for molecular nitrogen production involving the formation of an N2O intermediate. Despite many contributions from the experimental and theoretical studies on NO adsorption on the Rh6+ clusters and different surfaces, very little is known about the reaction mechanism for the conversion NO to N2 on the Rh7+ clusters; a description of detailed mechanisms at the atomic level is currently lacking and deserves special attention. On the basis of previous experimental results, herein, we present results from a density functional theory study of the decomposition of nitric oxide on the rhodium clusters, where the number of Rh atoms n = 7.
2. COMPUTATIONAL DETAILS The geometries of reactants, intermediates, transition states, and products were fully optimized via the Perdew Burke Ernzerhof35 (PBE) form of the generalized gradient approximation GGA. This functional was chosen since its reliable performance in describing Rh clusters is widely supported by the literature.36 39 The 6-311++G(d,P) basis set was chosen to describe the N and O atoms for the geometry optimization, and the Stuttgart Dresden (SDD)40 effective-core potential and valence Gaussian basis functions were considered for the rhodium atom. For each species, vibration analyses were performed to obtain the zero-point energies (ZPE) and verified whether each is a minimum or a transition state on the potential energy surfaces (PES). Intrinsic reaction coordinate (IRC) calculations were performed to confirm that a given transition state connects a particular couple of consecutive minima. All calculations were implemented in the Gaussian09 program.41 3. RESULTS AND DISCUSSION On the basis of previous experimental and theoretical results, a possible mechanism for the NO reduction on the Rh7+ cluster was proposed. In the present study, the first task is to investigate
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Figure 1. Side and top view of the optimized geometry of the cationic Rh7+ cluster. The value in parentheses is the multiplicity.
Figure 2. Potential energy surface for the adsorption and dissociation of the first NO molecule on the Rh7+ cluster.
which is the most likely structure and redox state of the cationic Rh7+ cluster. Previous calculations42 showed that the neutral Rh7 cluster has two low-energy isomers, that is, pentagonal bipyramid and capped octahedron clusters. In the case of the cationic Rh7+ cluster, according to the DFT calculation, the energy of the Rh7+ cluster with pentagonal bipyramid geometry lies 7.1 kcal/mol below the capped octahedron cluster. In addition, the geometry optimization of the Rh7+ cluster with spin multiplicities of 5, 9, and 11 are convergent. In contrast to the spin multiplicity of 11, the spin multiplicities of 5 and 9 have relatively higher energy than the spin multiplicity of 11 by 16.2 and 6.7 kcal/mol, respectively. As a result, the two state reactivity43,44 was not considered because of the system size and complexity, which would lead to a prohibitively large number of calculations. In the present calculations, the low-energy Rh7+ cluster with the spin multiplicity of 11 was taken into account as the initial reactant for the NO reduction reaction. Figure 1 shows the optimized geometry structure of the cationic Rh7+ cluster. Compared to that of the neutral structure, the cationic structure is not distorted due to the absence of the obvious Jahn Teller effect with the symmetry of D5h.42 As shown in Figure 1, the bond length of Rh6 Rh7 is about 2.525 Å, and the bond length of Rh1 Rh7 is equal to 2.629 Å. The first step of the reaction is the addition of a single NO molecule to the cationic naked Rh7+ cluster, then the N O bond breaks to form O and N atoms dispersed on the Rh7+ cluster. The potential energy profile of this reaction is displayed in Figure 2. As Figure 2 shows, substrate NO binds to the Rh7+ cluster in a tridentate fashion through its N atom; the strength of the binding is expected to give an indication of how easy it is to dissociate a NO molecule on the Rh7+ cluster since a greater adsorption energy would indicate stronger bonding between the metal Rh atoms and NO, which in turn should result in a weakening of the 14204
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The Journal of Physical Chemistry A N O bond. This is validated by the vibrational frequency analysis of the N O stretching mode (υNO) on the Rh7+ cluster and corresponds to the values of 1468 cm 1 in compound 1; the result suggested that a weaker N O bond was formed; this tresult is consistent with previous study.22,24 As displayed in Figure 1, the spin multiplicity of the Rh7+ cluster is 11. In the case of the addition of the NO molecule to the Rh7+ cluster, the spin multiplicity will change due to the doublet of the NO
Figure 3. Geometries for the adsorption and dissociation of the first NO molecule on the Rh7+ cluster. The values in parentheses are the multiplicities.
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molecule. According to our calculations, the geometry optimization of compound 1 with the spin multiplicities of 6, 10, and 12 are convergent. The spin multiplicity of 10 has a lower energy than the spin multiplicities of 6 and 12 by 7.7 and 4.5 kcal/mol, respectively. As shown in Figure 3, the spin multiplicity of 10 in compound 1 is used for the next calculations because of its relatively low energy. As Figure 3 displays, once the NO molecule binds to the Rh cluster, the Rh3 Rh7 bond breaks. The predicted bond length of the N O in compound 1 is about 1.219 Å, and the Rh3 N1, Rh7 N1, and Rh2 N1 bond lengths are calculated to be 1.938, 2.016, and 2.060 Å, respectively. The NO binds to the three-coordinated hollow sites of the Rh cluster is in agreement with the study by Borg.30 This step is necessary for the dissociation of the NO molecule. As shown in Figure 2, the formation of compound 1 (Rh7 NO+) is significantly exothermic by 45.3 kcal/mol; this is in agreement with the previous theoretical value of 50.3 kcal/mol related to the NO binding on the Rh(111) surface.21 The strong binding may facilitate the reduction of NO in the following step, and compound 1 constitutes the initial state in the catalytic reduction of NO. As displayed in Figure 2, compound 1 can lead to compound 2 via the transition state (TS12) with the activation barrier of about 39.6 kcal/mol. As Figure 3 shows, the N8 O9 distance in TS12 is equal to 1.830 Å. Compared to that of compound 1, the Rh7 N8 bond (2.037 Å) in TS12 is slightly lengthened, while the Rh3 N1 (1.886) and Rh2 N1 (1.937 Å) bond lengths decrease slightly. Also, it is worth noting that the distance between Rh3 and O9 is obviously decreased with the value of 2.945 Å. The only
Figure 4. Potential energy surface of the alternative reaction path for the adsorption and dissociation of the second NO molecule on the [Rh7NO]+ cluster. 14205
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Figure 5. Geometries for the adsorption and dissociation of the second NO molecule in reaction path I on the [Rh7NO]+ cluster. The values in parentheses are the multiplicities.
imaginary frequency of 452i cm 1 confirms the saddle-point character of the transition state, and it corresponds to the dissociation of the N8 O9 bond. The N8 O9 bond at TS12 begins to break and yields compound 2 with a slightly endothermic value of 2.9 kcal/mol, indicating that this step is energetically favored. As displayed in Figure 3, the N8 atom in compound 2 is dispersed on the Rh2, Rh3, and Rh7 atoms, while the O9 atom is dispersed on the Rh3 atom, and the Rh3 Rh7 bond is newly generated. Furthermore, the yield of the more stable compound 3 requires exothermicity of 12.7 kcal/mol. In contrast to compound 2, the O9 atom in compound 3 can both bind Rh3 and Rh4 atoms, leading to the rupture of the Rh3 Rh7 bond. According to Figure 2, the breaking of the N O bond is energetically favorable with the whole exothermicity of about 55.1 kcal/mol. After the formation of the [Rh7NO]+ cluster, the second NO molecule will react with this cationic cluster. Then, two NO molecules are coadsorbed on this Rh cluster, and N2 is evolved, yielding the corresponding dioxide cluster. In the mechanism discussed later, this reaction can take place in two alternative ways. The reaction path initiated by NO binding to the N9 atom is called reaction path I; the potential energy surface is described in Figure 4. The reaction path initiated by NO binding to the Rh7 atom is called reaction path II, the potential energy surface is also represented in Figure 4. In addition, the geometries for the adsorption and dissociation of the second NO molecule in the
Table 1. Relative Energy (RE; in kcal/mol) for Each Intermediate from the NO Reduction Reaction in an Alternative Reaction Path reaction species +
Rh7 + NO 1
reaction
RE
path I
RE
path II
RE
0.0 45.3
3 + NO 4
0.0 14.6
3 + NO 8
0.0 45.8 11.9
TS12
5.7
TS45
2.5
TS89
2
42.4
5
19.5
9
3
55.1
TS56
13.6
TS910
9.2 9.4
6
16.7
10
62.4
TS67
15.1
7 + N2
40.8
7 + N2
43.2
reaction paths I and II on the [Rh7NO]+ cluster are summarized in Figures 5 and 6. For reaction path I, compound 3 adsorbs the second NO molecule to produce compound 4. As shown in Figure 5, the interaction between the N atom in the second NO and the N8 atom in compound 3 are not very strong; this process releases the energy of only 14.6 kcal/mol (Table 1). In the next step, the two N atoms are coupled with each other via TS45, and the predicted activation barrier is about 12.1 kcal/mol, which is the rate-determining step in reaction path I and can be strongly driven by a large exothermicity of the reaction. As 14206
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Figure 6. Geometries for the adsorption and dissociation of the second NO molecule in reaction path II on the [Rh7NO]+ cluster. The values in parentheses are the multiplicities.
Figure 5 shows, the N N distance in TS45 is about 1.638 Å. After the formation of the N N bond, compound 5 is formed with the exothermic value of about 4.9 kcal/mol; this step is validated by Zaera and co-workers.31 In this compound, the N8 atom in N2O can bind both Rh3 and Rh7 atoms. Compound 5 can then produce compound 6 (bridge-mode) via TS56 with the endothermic value of about 2.8 kcal/mol. The predicted activation barrier for the formation of the Rh 7 O bond in TS56 is only about 5.9 kcal/mol as shown in Figure 4. The Rh7 O bond length in TS56 is calculated to be 2.605 Å. The N O bond of N2O on the Rh cluster breaks and produces the final product N2 and a partially oxidized rhodium cluster (compound 7). This step goes over a relatively low barrier (TS67) of only 1.6 kcal/mol; a present result is in well agreement with previous calculated value on the Rh6+ cluster performed by Hamilton.28 The reaction [Rh7NO]+ + NO f N2 + Rh7O2+ involved in the intermediate N2 O releases the energy of about 43.2 kcal/mol. In addition, for this reaction, the N2 is originally from the decomposition of N2O; these results are in well agreement with previous experiments.31 34 For the reaction path II, as shown in Figure 6, the second NO binds to the Rh7 atom to produce compound 8, with the exothermicity of about 45.8 kcal/mol. In the next step, the N O bond breaks to yield compound 9 with the isolated N and O atoms bonded to different Rh2 and Rh6 atoms, respectively. As Figure 4 depicts, the activation barrier is predicted to be 57.7 kcal/mol via the transition state TS89; this is the rate-limiting step of reaction path II. The large reaction barrier indicates that this step is unfavored kinetically. Then, the two N atoms couple with each other via TS910 with the reaction barrier of about 18.6 kcal/mol. The formation of compound 10 releases the energy of 53.2 kcal/mol. Ultimately, the N2 is released from the Rh cluster to form compound 11 with the endothermicity of about 21.6 kcal/mol. On the basis of present calculations, in
contrast to reaction path II, reaction path I is energetically more favorable.
4. CONCLUSIONS The reaction mechanism of NO reduction on the Rh7+ cluster has been investigated by means of DFT calculations. The reaction can proceed via three steps. The first step is related to the reaction Rh7+ + NO f [Rh7NO]+, and the next step corresponds to the decomposition of NO to yield N and O atoms dispersed on different Rh atoms. The last step is involved in the reaction [Rh7NO]+ + NO f N2 + Rh7O2+. Our calculations show that the first NO molecule dissociates on the Rh7+ cluster requiring the activation barrier of 39.6 kcal/mol, which is the rate-determining step of the whole catalytic cycle, and yields the compound [Rh7NO]+ with N and O atoms dispersed on different Rh atoms. This step can be driven via the large exothermic value of 53.2 kcal/mol. For the second NO adsorption, the reaction can take place via two alternative ways. Reaction path I is involved in the intermediate N2O being adsorbed on the Rh cluster, and reaction path II is involved in two isolated N atoms straightly coupled with each other to form N2. Our calculations show that reaction path I has a relatively low activation barrier and is energetically favored. The present work is in good agreement with previous research results.25,31 ’ ASSOCIATED CONTENT
bS
Supporting Information. Optimized Cartesian coordinates for all species in the present study. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +86 571 88053832. Fax: +86 571 88053832. E-mail: hujunxie@ gmail.com. 14207
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’ ACKNOWLEDGMENT We acknowledge financial support from the National Science Foundation of China (21073164 and 20673098), the Natural Science Foundation of Zhejiang Province (Y4100620), and the Research Foundation of Education Bureau of Zhejiang Province (Y200906517). We thank the State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University) for providing computational resources. ’ REFERENCES (1) Taylor, K. C. Catal. Rev. Sci. Eng. 1993, 35, 457. (2) Shelef, M.; Graham, G. W. Catal. Rev. Sci. Eng. 1994, 36, 433. (3) Liu, S. W.; Horino, H.; Kokalj, A.; Rzeznicka, I.; Imamura, K.; Ma, Y. S.; Kobal, I.; Ohno, Y.; Hiratsuka, A.; Matsushima, T. J. Phys. Chem. B 2004, 108, 3828. (4) Taylor, K. C. In Catalytic Chemistry of Nitrogen Oxides; Klimisch, R. L., Larson, J. G., Eds.; Plenum: New York, 1975; p 173. (5) Shelef, M.; Graham, G. W. Catal. Rev. Sci. Eng. 1994, 36, 433. (6) Schmatlocn, V.; Kruse, N. Surf. Sci. 1992, 269/270, 488. (7) Obuchi, A.; Ohi, A.; Nakamura, M.; Ogata, A.; Mizuno, K.; Obuchi, H. Appl. Catal., B 1993, 2, 71. (8) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 75, L1. (9) Brown, W. A.; King, D. A. J. Phys. Chem. A 2000, 104, 2578. (10) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283. (11) Parvulescu, V. I.; Grange, P.; Delmon, B. Catal. Today. 1998, 46, 233. (12) Teffo, J. L.; Chedin, A. J. Mol. Spectrosc. 1989, 135, 389. (13) Zeigarnik, A. V. Kinet. Catal. 2003, 44, 233. (14) Schmatlock, V.; Jirka, I.; Kruse, N. J. Chem. Phys. 1994, 100, 8471. (15) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal., B 2002, 39, 283. (16) Taylor, K. C.; Schlatter, J. C. J. Catal. 1980, 63, 53. (17) Loffreda, D.; Simon, D.; Sautet, P. J. Catal. 2003, 213, 211. (18) Inderwildi, O. R.; Lebiedz, D.; Deutschmann, O.; Warnatz, J. J. Chem. Phys. 2005, 122, 154702. (19) Loffreda, D.; Delbecq, F.; Simon, D.; Sautet, P. J. Chem. Phys. 2001, 115, 8101. (20) Loffreda, D.; Simon, D.; Sautet, P. J. Chem. Phys. 1998, 108, 6447. (21) Mannstadt, W.; Freeman, A. J. Phys. Rev. B 1997, 55, 13298. (22) Torres, M. B.; Aguilera-Granja, F.; Balbas, L. C.; Vega, A. J. Phys. Chem. A 2011, 115 (30), 8350–8360. (23) Stirling, A.; Gunji, I.; Endou, A.; Oumi, Y.; Kubo, M.; Miyamoto, A. J. Chem. Soc., Faraday Trans. 1997, 93, 1175. (24) Ghosh, P.; Pushpa, R.; de Gironcoli, S.; Narasimhan, S. J. Chem. Phys. 2008, 128, 194708. (25) Anderson, M. L.; Ford, M. S.; Derrick, P. J.; Drewello, T.; Woodruff, D. P.; Mackenzie, S. R. J. Phys. Chem. A 2006, 110, 10992. (26) Ford, M. S.; Anderson, M. L.; Barrow, M. P.; Woodruff, D. P.; Drewello, T.; Derrick, P. J.; Mackenzie, S. R. Phys. Chem. Chem. Phys. 2005, 7, 975. (27) Hamilton, S. M.; Hopkins, W. S.; Harding, D. J.; Walsh, T. R.; Haertelt, M.; Kerpal, C.; Gruene, P.; Meijer, G.; Fielicke, A.; Mackenzie, S. R. J. Phys. Chem. A 2011, 115, 2489. (28) Hamilton, S. M.; Hopkins, W. S.; Harding, D. J.; Walsh, T. R.; Gruene, P.; Haertelt, M.; Fielicke, A.; Meijer, G.; Mackenzie, S. R. J. Am. Chem. Soc. 2010, 132, 1448. (29) Harding, D.; Ford, M. S.; Walsh, T. R.; Mackenziew, S. R. Phys. Chem. Chem. Phys. 2007, 9, 2130. (30) Borg, H. J.; Reijerse, J. F. C.-J. M.; van Santen, R. A.; Niemantsverdriet, J. W. J. Chem. Phys. 1994, 101, 10052. (31) Zaera, F.; Gopinath, C. S. Chem. Phys. Lett. 2000, 332, 209. (32) Zaera, F.; Gopinath, C. S. Phys. Chem. Chem. Phys. 2003, 5, 646.
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