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J. Phys. Chem. B 2004, 108, 12264-12266
Selective Catalytic Reduction of Nitric Oxide by Ammonia: The Activation Mechanism
Yuka Kobayashi,* Nobuo Tajima, Haruyuki Nakano, and Kimihiko Hirao
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo,Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-8656 Japan ReceiVed: May 12, 2004; In Final Form: June 25, 2004 The activation mechanism of NH3 in the selective catalytic reduction of NO by NH3 on a V2O7H4 cluster wasinvestigated using a complete active space self-consistent field method. Because of the easy bond dissociationof NH + adsorbed on Brønsted acid sites of the V2O5 configuration, the radical species NH3 can occur with an activation energy of only 26.7 kcal/mol. The highly active intermediate NH + very strong hydrogen bond of approximately 29.2 kcal/mol to the vanadyl oxygen. This stabilization mechanismis very similar to the low-barrier hydrogen bond in the transition state, or in an unstable intermediate state,which has been reported for some enzymatic reactions.
In recent decades, the problem of air pollution and acid rain, perturbation method.11 In that study, a large difference in the caused by toxic gases such as NOx and SOx, has become energy profile of the initial NsN bond formation step was ecologically serious.1 The technique of decomposing NO observed. In the first NsN bond intermediate produced by NH4 the selective catalytic reduction (SCR) of NO by NH3 over and NO, the NsN bond is very unstable because one electron vanadia-based catalysts is very effective. This method, which occupies its antibonding orbital. Therefore, a large amount of was established in the 1970s, is still the major strategy for the energy (71.4 kcal/mol) is required to form the NsN bond reduction of NO industrially. Therefore, considerable effort has complex, which indicates a low reactivity. Conversely, NO been spent in elucidating the reaction processes of the SCR, exothermically and forms a relatively stable and many different schemes have been suggested in the NsN coordination bond, where the radical electron is transferred literature.2 However, the development of a next-generation 3 . The resulting complex is produced without catalyst demands a deep understanding of the reaction mech- NO efficiently on the V2O5 surface. However, in the gas phase, NH3 has a much higher total energy than NH4 . In the present adsorbed on a V2O5 surface. In contrast, NH3 has been reported study, we theoretically investigated these total energies at the to be readily absorbed on the surface of Brønsted acid sites of V2O5.3,4 These observations lead to the conclusion that the SCR Recently several groups are investigating the behavior of reaction can be described by the Eley-Rideal mechanism on the catalyst. A wide range of adsorption energy of NH4 is proposed on the basis of various surface models, e.g., experimental evidence has been that both the VsOH (Brønsted VO3H3, V2O7H4, V2O9H8, and V4O16H12, etc., using various acid sites) and the VdO species of a V2O5 surface are crucial theoretical methods.12 However, crystal structure shows that the for conversion of NOx.6 In situ Fourier transfrom IR (FTIR) coordination environment of the V atom is tetrahedral and that and on-line mass spectroscopic studies have shown that NH + the oxidation number is +5.13 The simple V2O7H4 cluster model is the predominant species on vanadia/titania catalysts under is more reasonable to describe the (010) surface of the V2O5 SCR conditions.7 However, no direct observation of a NO- crystal, although this model has never been reported successful reacting species has been reported in any experiment. The identification of the active species is clearly one of the most this system originates from the VdO double bond. A compli- important, and challenging, subjects left in order to explain the cated electronic structure arises because of the d orbitals of the reaction mechanism of SCR. NH3 adsorbed at Brønsted acid V atom. A complete active-space self-consistent field method (CASSCF) represents the π-bonding system in a manner that that the adsorbed ammonia never interacts directly with NO.
allows the electrostatic correlation to be treated with high At high temperatures (>300 °C), NH2 is considered to be the accuracy.14 The breakdown of the mean-field approximation, activated species in the thermal decomposition of NO.8 Because which was observed in applying Hartree-Fock (HF) and density NH2NO, which is produced when NH2 and NO combine, has functional theory (DFT), can be avoided. We herein demonstrate never been detected under SCR conditions on a VdO, it cannot the adsorption energy and possibility of the existence of unstable intermediate, NH3 , on the surface using V2O7H4 model by suggested as the activated species in SCR.10,6 Recently, our means of the method based on multiconfiguration.
group investigated the reactivity of NH + under gas-phase conditions using a second-order multireference Computational Details
* To whom correspondence may be addressed. E-mail: [email protected] All geometries, which are not defined in the text, were tokyo.ac.jp. Fax: +81-3-5841-7267. Current address: Department of optimized using 3-21G basis sets.15 The determined structures Chemistry and Biotechnology, School of Engineering, The University ofTokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.
were evaluated using the following basis sets: SBKJC VDZ J. Phys. Chem. B, Vol. 108, No. 33, 2004 12265
Figure 1. The adsorption structures of NH +
and NH3 on a V2O7H4 cluster are shown in (a) and (b), respectively. Bond lengths are shown in angstroms, and the angles in parentheses show experimental values. Bold arrows mean electron spins and hushed lines show hydrogen bonds. (a)The NH + adsorbs in a nearly symmetrical fashion at the two VdO sites (Brønsted acid sites) by hydrogen bonds. (b) After dissociation, the NH3 stands on the VdO site with a strong hydrogen bond.
ECP plus polarization for the V atoms;16 cc-pVDZ for the O TABLE 1: Main and Minor Configurations in
CASSCF[6e,9o] Wavefunctions of the V2O7H4 Cluster in
atoms of VdO sites;17 cc-pVDZ for the N and H atoms of the Singlet and Triplet States
and NH3 ; 3-21G for all other atoms. For the V2O7H4 cluster, the active space was chosen as [8e,8o], which includes 2 pairs of dπ orbitals for each VdO site. For the supermoleculeof NH + on the V2O7H4 cluster, [12e,12o], namely, [4e,4o] for VdO, and [8e, 8o] for the two VdO sites was chosen. All calculations were performed using the ab initio quantum dσ(2) f dσ*(1)dπ(2) f dπ*(1) Results and Discussion
To examine the bonding property of VdO, we calculated 2O7H4 cluster with CASSCF[6e,9o] /3-21G using the structures, which is determined by ROHF/3-21G in singlet and state is more stable as 38.5 kcal/mol than that in the triplet state.
This calculation clarified that VdO bond has quite multicon- figurational character in both states. The weight of the main electronic configuration is only 85%; the rest of the configu- a The occupied electron numbers are shown in parentheses. b Pa- ration consists of dπ-dπ* and dσ-dσ* contributions, even in rentheses mean the excitation electron numbers from the main the ground state, as shown in Table 1. The bond length of the configurations. For example, dπ(2) f dπ*(1) means one electron terminal vanadyl groups (VdO) was reoptimized with CASSCF- excitation configuration from doubly occupied dπ orbital to dπ*orbital.
[8e,8o]/3-21G and found that it was 1.63 Å. This is marginallylonger than the experimental value for a V2O5 single crystal kcal/mol). This corresponds to the required energy for the (1.58 Å), measured by X-ray diffraction.13 Theoretical investiga- in the gas phase, on an obviously unstable tions on vanadyl, which is 3-fold and 4-fold coordinated with oxygen, show only marginal changes in the VdO bond length.
The difference of adsorption energy between the experimental and theoretical results is a good measure for estimating the with CASSCF[12e,12o] using basis sets mentioned in the quality of a calculation. CASSCF[12e,12o] yields E computational section. The optimized structures and the energy cluster, which is in good agreement with 18-26 kcal/mol, in Figures 1 and 2, respectively. The theoretical representation observed in the experiment.20 On the other hand, DFT gives too large adsorption energy for V2O7H4; the values are in ∼50- an open-shell singlet state (Figure 1b). There is no activation 60 kcal/mol, although it gives a reasonable value for V2O9H4, barrier, which is similar to the result in the gas phase. The energy difference between the reactant and the product is Ediss(V2O7H4) The potential-energy surface (PES) of the reaction NH + 51.4 kcal/mol. Although zero-point energy and entropic H in the gas phase was investigated using CASSCF- effects are not included in this calculation, these effects would [8e,8o]/cc-pVDZ prior to the surface reaction.21 This revealed not have a strong influence to the result. In a solid catalytic that the potential energy increased monotonically with the NH + system, the real activation energy of the reaction is reduced by to H distance, that there was no transition state to produce NH + the heat of adsorption of the molecules on the catalytic surface and also that the energy difference between the reactant NH + (see Figure 2).22 Under SCR conditions, NH4 adsorption species.10 It can be considered that most of its 12266 J. Phys. Chem. B, Vol. 108, No. 33, 2004
example, in enzymes.25 In several enzymatic reactions, a LBHBis formed in the enzyme-substrate complex at a transition stateand/or an intermediate, where it is considered that an energy of10-20 kcal/mol is saved, the reaction is therefore significantlyassisted. For example, on vanadium haloperoxidases, it has beensuggested that the hydrogen bond formed between a V-O siteand its coordinated amino residues is significant in the catalyticmechanism.26 In this report, the energy profile of the key molecule NH + on the V2O5 surface in SCR was derived using an accurate abinitio molecular orbital method. The multiconfigurational char-acter of the VdO site is suitable to extract a hydrogen atomfrom the absorbed NH + hydrogen affinity. Because of the contribution of the LBHBenergy to the activated VdO site, only 26.7 kcal/mol is requiredto produce an activated NH + in SCR was revealed, and the beautiful similarity between theSCR and an enzymatic reaction mechanism with a LBHB wasshown.
Figure 2. The energy diagram of the species on the V2O7H4 model,
calculated with CASSCF[12e,12o]/SBKJC VDZ ECP plus polarization
References and Notes
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(2) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal. B 1998, 18,
molecule in the gas phase without any interaction to the V2O7H4. State 2 represents the adsorbed NH4 which relates to Figure 1a. Moving to (3) Mitamoto, A.; Yamazaki, Y.; Hattori, T.; Inomata, M.; Murakami, state 3, the V2O7H4 abstracts one hydrogen atom from the NH4 . (Figure Y. J. Catal. 1982, 74, 144.
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