14424
J. Phys. Chem. 1996, 100, 14424-14429
Theoretical Study of NO Decomposition on Cu-ZSM-5 Catalyst Models Using the Density Functional Method Yasunori Yokomichi* and Tokio Yamabe DiVision of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto 606-01, Japan
Hirofumi Ohtsuka and Terumitsu Kakumoto Fundamental Research Laboratories, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 554, Japan ReceiVed: February 8, 1996; In Final Form: June 3, 1996X
The decomposition mechanisms of NO on Cu-ZSM-5 catalyst models were theoretically investigated. The structure and frequency of adsorbed NO on Cu-ZSM-5 and the poisoning of Cu-ZSM-5 by oxygen can be reproduced by the calculations using the density functional method with Huzinaga’s MIDI+P basis set.
Introduction It is reported that metal-ion-exchanged zeolites such as Cu-ZSM-5 show certain activities for the decomposition of NO and are rapidly deactivated by the oxygen resulting from decomposed NO.1 The experimental results have suggested that the Cu+ site is active for NO decomposition in Cu-ZSM5.2 However, hardly any theoretical studies on this reaction system have been reported except for a few papers.3,4 In our previous paper,5 it was suggested that Hartree-Fock (HF) level calculations disregarding electron correlations were insufficient to reproduce NO adsorption and decomposition on CuZSM-5. In this report, we describe the results of our investigation on the reaction mechanism in which the calculations of the reaction center are based on an ab initio molecular orbital method regarding electron correlation such as the density functional method (DFT). Models and Computational Details All calculations were carried out on a Kubota Titan 2-800 workstation, DEC 3000-800 workstation and a Fujitsu VPX210/ 10 supercomputer using Gaussian 92/DFT and Gaussian 946 program packages. As a preliminary evaluation of calculation methods to select a suitable DFT method for this study, several calculations on N2O4 (listed in Table 1) were carried out using the MøllerPlesset method (MP2), Becke’s 1988 exchange functional with Perdew’s gradient-corrected correlation functional method (BP86), Becke’s half-and-half exchange functional with Lee, Yang, and Parr correlation functional method (BH&HLYP), Becke’s 1988 exchange functional with Lee, Yang, and Parr correlation functional method (BLYP), and Becke’s threeparameter exchange functional with Lee, Yang, and Parr correlation functional method (B3LYP) by the 6-31G* basis set. After this preliminary evaluation, the BP86 method was selected for the study of Cu-ZSM-5 catalyst models. The frequency and dissociation energy of NO, N2, and O2 were calculated to confirm the accuracy of the BP86 method. All calculations on catalyst models shown in Figures 1 and 2, where the zeolite skeleton has been simplified to Al(OH)4 anion and Cu+ was used as an active site, were performed with X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00403-0 CCC: $12.00
TABLE 1: Results of Calculations Using MP2 and DFT Methods on a N2O4 Molecule bond length of N-N (Å) observed value MP2/6-31G* BP86/6-31G* BH&HLYP/6-31G* BLYP/6-31G* B3LYP/6-31G*
frequency (cm-1)
1.75 1.82 1.85 1.67 1.89 1.78
1373 1412 1409 1533 1381 1330
1710 1948 1757 1943 1704 1830
1748 1980 1779 1974 1725 1857
TABLE 2: Accuracy of BP86 on Some Basic Molecules frequency (cm-1)
dissociation energy (kcal/mol)
molecule
observed
calculated
observed
calculated
NO N2 O2
1875.9 2330.7 1554.7
1875 2312 1537
149.71 224.90 117.89
166.15 232.94 139.82
Huzinaga’s uncontracted MIDI+P basis set (Cu: 53321/521/ 41, N: 521/41/1, O of NO: 521/41/1, Al: 5321/521, O: 521/ 41, H: 41).7 The scale factor of the d orbital is 1.0 for the N atom and O atom of the NO molecule. The geometry of II, VII, and VIII was fully optimized while keeping C2V symmetry because of their lower energy than that of nonsymmetric structures. The calculation that started from a 3-fold coordination of model II also gave the C2V structure after full geometry optimization. The stability of the wave functions was checked on all the models in multiplet state. In calculations III-VI, the fixed structure of catalyst model II was used. To investigate poisoning by oxygen, a small cluster model consisting of two copper ions was used. Model VIII was set at the same number of electrons as model IX to evaluate the adsorption energy of oxygen in comparison to these models. It is assumed that model VIII is a result of model IX by the desorption of oxygen anion as oxygen radical. The energy changes between each optimized structure (ground state) were evaluated by comparing the total energies in order to study qualitatively the decomposition mechanisms of NO on Cu-ZSM-5. Results and Discussion Accuracy of the BP86 Method. As shown in Table 1,8 B3LYP is the most accurate geometrically but is not as accurate © 1996 American Chemical Society
NO Decomposition
J. Phys. Chem., Vol. 100, No. 34, 1996 14425
Figure 1. Models used for calculations of NO adsorption and decomposition on Cu-ZSM-5.
Figure 2. Models used for calculations of poisoning by oxygen on the reaction center.
as BP86 and BLYP for the frequency of the N2O4 molecule. BLYP is better than BP86 on frequency but worse geometrically. BP86 is well balanced in terms of geometric accuracy and frequency. BP86 is also accurate in calculations of the frequency and the dissociation energy of basic molecules shown in Table 2.9 NO Adsorption on Cu-ZSM-5 Catalyst Model. Although all forces were less than 10-5 hartrees/bohr, the optimized structure of model II had one imaginary frequency (142i cm-1) assigned to the out-of-plane bending mode of OH
(a2 symmetry). The results of our calculations indicate that the NO molecule is adsorbed on Cu-ZSM-5 catalyst model II with the nitrogen atom facing the copper atom and the adsorption energy is -51 kcal/mol (Table 3 and Figure 3). The frequency of the NO molecule shifts from 1875 cm-1 (free NO) to 1815 cm-1 (adsorbed NO) without scaling (Table 4). These results agree with the experimental data,10 especially regarding the frequency shift. The changes of the structure and the atomic charges in models I, II, and III-a explain that an isolated copper ion does not have any activity in NO decomposition, and the
14426 J. Phys. Chem., Vol. 100, No. 34, 1996
Yokomichi et al.
TABLE 3: Results of Calculations on NO Adsorption Using BP86 model
NO
Cu+
I
II
III-a
III-b
IV
V
VI
bond length (Å) Cu-O 1.813 Cu-N 1.870 1.739 1.741 1.958a 1.918 N-O 1.174 1.160 1.190 1.207 1.191a 1.218 atomic charge Cu 1.000 0.901 0.747 0.986 0.936 1.031 0.921 0.908 Al 1.506 1.524 1.520 1.523 1.523 1.524 N 0.006 -0.069 -0.234 -0.036 -0.340 -0.132a O of NO -0.006 0.168 -0.065 -0.199 -0.009a -0.144 atomic spin density Cu 0.000 0.230 0.000 0.031 -0.073 0.527 -0.008 0.000 A1 0.000 0.000 0.001 0.005 0.003 0.000 N 0.692 0.437 0.613 0.860 2.384 0.582a 0.000 O of NO 0.308 0.333 0.362 0.237 0.435a 0.000 S2 (before annihilation) 3.752 0.000 0.754 0.000 0.755 0.785 3.754 2.041 0.000 S2 (after annihilation) 3.750 0.000 0.750 0.000 0.750 0.750 3.750 2.000 0.000 total energy (hartree) -129.8884 -1639.9932 -1769.9415 -2186.0878 -2316.0574 -2316.0228 -2240.7422 -2445.9754 -2370.8061 NO adsorption energy -37.59 -50.95 -29.24 -69.53b -29.88c (kcal/mol) a
Averaged value. b For two NO molecules. c For N2O molecules.
Figure 3. Optimized structure of NO-adsorbed catalyst models (italic: atomic charges). Bond lengths are given in angstroms and bond angles in degrees.
TABLE 4: Frequency of Adsorbed NO on Cu-ZSM-5 Catalyst Model Using BP86 without Scaling free NO
adsorbed NO (III-a)
adsorbed NO (V) -1)
1875
1875.9
Calculated Frequency (cm 1815
1748 (asym) 1642 (sym)
Observed Frequency10 (cm-1) 1807-1815 1825 (asym) 1732 (sym)
copper ion loaded in the zeolite skeleton destabilizes the NO molecule by transferring the electron from the copper atom to the NO molecule.
Furthermore, it is found that the d electrons of copper atoms are destabilized and activated by interaction with the oxygen of the zeolite skeleton from the comparative study of the orbitals of Cu+ and II. The HOMO level of II rises from -0.47 hartree (degenerate d orbital of Cu+) to -0.14 hartree. Because the HOMO level of II is close to the LUMO level of a NO molecule and the symmetry of these orbitals is good for interaction, a strong interaction between these orbitals occurs. The HOMO and LUMO levels of the NO-adsorbed model III-a are lower than the HOMO of II and the LUMO of NO, and the electron is transferred from the HOMO of II to the LUMO of the NO molecule (Figure 4).
NO Decomposition
J. Phys. Chem., Vol. 100, No. 34, 1996 14427
Figure 5. Schematic energy profile for NO adsorption and decomposition on a catalyst model. Energies are given in kcal/mol.
Figure 4. Scheme of interactions between orbitals of NO and CuZSM-5 catalyst model.
The calculation on model V can reproduce the frequency of the twin NO-adsorbed state experimentally observed within 5% error (Table 4), and both of the NO molecules are destabilized for the reason mentioned above. Reaction Route of NO Decomposition and Poisoning by Oxygen. The dissociation energy of NO on the model III-a is calculated as 164.2 kcal/mol (Figure 5), but this value is too large to explain the experimental result that NO on Cu-ZSM-5 is decomposed at room temperature. This inconsistency indicates that models including only one NO or one copper atom are too small to explain the experimental results. The results of the calculation also indicate that the NO-adsorbed energy of III-a is -51.0 kcal/mol and the stabilized energy from III-a to V is -18.5 kcal/mol. This suggests that all the Cu+ sites in Cu-ZSM-5 are covered by one molecule of NO at first and then the remaining free NO can be adsorbed as secondary NO. Since the enthalpy of the decomposition of the adsorbed twin NO is very large, this reaction cannot progress spontane-ously
without the participation of the other molecules such as the oxygen molecule utilizing the formation energy for progressing the reaction (calculated formation energy of O2 is -139.8 kcal/mol). As described above, although the results of calculations on V using BP86 show good accuracy, the decomposition of NO from twin NO cannot be explained (Figure 6). However, the calculated adsorption energy of atomic oxygen on model II is -79.0 kcal/mol (Table 5) and larger than half the formation energy of O2 (-69.9 kcal/ mol) and the NO adsorption energy on model II (-51.0 kcal/mol). These results suggest that the catalyst model II adsorbing atomic oxygen cannot adsorb NO and that atomic oxygen tends to be adsorbed rather than to form an O2 molecule with another oxygen atom. The calculations on the catalyst model II can reproduce the poisoning of Cu-ZSM-5 catalyst by oxygen. Since the calculations on model III-a and V cannot explain the reaction route of NO decomposition, calculations on models VIII-XI were carried out to evaluate the adsorption energy of atomic oxygen on the two-center model IX, which is a calculation model of copper dimer existing in Cu-ZSM-5 (Table 5 and Figure 7). The optimized geometry of models VII and IX show that atomic oxygen has properties like O- or O2anions after adsorption on Cu-ZSM-5 and the copper cluster. Although the calculations indicate that the adsorption energy
TABLE 5: Results of Calculations of Atomic Oxygen and NO Molecule Adsorption on a Copper Small Cluster Model Using BP86 model bond length (Å) Cu-O (adsorbed) Cu-N N-O atomic charge Cu O (adsorbed) N of NO O of NO atomic spin density Cu O (adsorbed) N of NO O of NO S2 (before annihilation) S2 (after annihilation) total energy (hartree) adsorption energy (kcal/mol)
O atom
VII
VIII
1.730
1.058 -0.413
2.000
2.002 2.000 -75.0535
0.530 1.326
2.005 2.000 -2261.2672 -79.00
IX
X-a
1.844
0.778
0.000
0.000 0.000 -3356.2150
1.072 -0.586
0.396 1.052
2.005 2.000 -3431.3677 -62.25
X-b 2.388
1.956 1.191
1.199
0.930
0.839
-0.375 0.059
0.049 -0.186
0.129
-0.017
0.334 0.302 0.753 0.750 -3486.1446 -25.85
0.795 0.252 0.761 0.750 -3486.1081 -2.95
XI 3.130
0.915 -0.296, +0.013
0.161 0.634, 0.945
2.007 2.000 -3506.5605 -9.85
14428 J. Phys. Chem., Vol. 100, No. 34, 1996
Yokomichi et al.
Figure 6. Schematic route of NO decomposition on Cu-ZSM-5 catalyst model via twin adsorption and poisoning by oxygen. Energies are given in kcal/mol.
Figure 7. Optimized structure of small cluster catalyst models (italic: atomic charges). Bond lengths are given in angstroms and bond angles in degrees.
of atomic oxygen on model IX (-62.3 kcal/mol) is larger than those of NO (-25.9 kcal/mol) and that atomic oxygen can prevent a NO molecule from adsorbing on the Cu+ site, oxygen atoms resulting from the decomposition of NO prefer to make O2 rather than be adsorbed on Cu+ sites because half the formation energy of O2 (-69.9 kcal/mol) is larger than the
adsorption energy of atomic oxygen on model IX (-62.3 kcal/ mol, Figure 8). The results of our investigation suggest that a large calculation model including more than two reactants and copper atoms is necessary to reproduce the reaction mechanism of NO decomposition on Cu-ZSM-5.
NO Decomposition
J. Phys. Chem., Vol. 100, No. 34, 1996 14429
Figure 8. Schematic route of atomic oxygen on a copper small cluster model. Energies are given in kcal/mol.
Conclusions
References and Notes
1. The molecular orbital calculations using the density functional method (BP86) can reproduce the frequency of NO adsorbed on Cu-ZSM-5 without scaling. 2. The calculations indicate that Cu is activated by the interaction between Cu and the zeolite skeleton. 3. Calculations using BP86 on Cu-ZSM-5 catalyst model II can reproduce the poisoning by oxygen. 4. The calculations on the Cu-ZSM-5 catalyst model and the two center cluster model cannot reproduce the decomposition route of NO adsorbed on Cu-ZSM-5, strongly suggesting that the large calculation model including more than two reactants and copper atoms is necessary.
(1) Iwamoto, M.; Yahiro, H. Catal. Today 1994, 22, 5-18. (2) Lei, G. D.; Adelman, B. J.; Sarkany, J.; Sachtler W. M. H. Appl. Catal. B 1995, 5, 245-256. (3) Hrusak, J.; Koch, W.; Schwarz, H. J. Chem. Phys. 1994, 101, 3898-3905. (4) MaKee, M. L. J. Am. Chem. Soc. 1995, 117, 1629-1637. (5) Yokomichi, Y.; Ohtsuka, H.; Tabata, T.; Okada, O.; Yokoi, Y.; Ishikawa, H.; Yamaguchi, R.; Matsui, H.; Tachibana, A.; Yamabe, T. Catal. Today 1995, 23, 431-437. (6) Frisch, M.; Foresman, J.; Frisch, A. Gaussian 92/DFT; Gaussian Inc.: Pittsburgh, PA, 1992. Frisch, M.; Foresman, J.; Frisch, A. Gaussian 94; Gaussian Inc.: Pittsburgh, PA, 1994. (7) Huzinaga, S. Gaussian Basis Sets for Molecular Calculations; Elsevier: Amsterdam, 1984. (8) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985, 14, 1560. (9) Herzberg, G. Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules, 2nd ed.; D. van Nostrand Co. Inc.: Princeton, NJ, 1950. (10) Valyon, J.; Hall, J. K. J. Phys. Chem. 1993, 97, 1204.
Acknowledgment. This study was performed as a part of the national project “Development of Ceramic Gas Engine” coordinated by The Japan Gas Association, which administered the project with the financial support of the Japanese government (Ministry of International Trade and Industry).
JP960403E
