J. Phys. Chem. 1996, 100, 11649-11653
11649
FT-IR and Quantum Chemical Studies of the Interaction between Dimethyl Ether and HZSM-5 Zeolite T. Fujino, M. Kashitani, J. N. Kondo, K. Domen, and C. Hirose* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
M. Ishida and F. Goto Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., 6 Kitahara, Tsukuba, Ibaraki 300-32, Japan
F. Wakabayashi Department of Science and Engineering, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169, Japan ReceiVed: October 24, 1995; In Final Form: February 27, 1996X
Adsorption of dimethyl ether (DME) on the protonated ZSM-5 zeolite was investigated by FT-IR spectroscopy and density functional theory (DFT) calculation. The coverage and temperature dependencies of characteristic bands and the adsorption on the partially deuterated zeolite were studied by FT-IR. Three bands observed at ca. 2900, 2400, and 1600 cm-1 were ascribed to the bands associated with the Brønsted acidic OH groups hydrogen-bonded to DME, and the band at ca. 3300 cm-1 was assigned to the OH stretching mode of the silanol OH groups hydrogen-bonded to DME. Both the IR and DFT results indicated that DME molecules adsorbed on OH groups by a hydrogen bonding irrespective of the acidity of the OH groups and that the oxonium ions of DME were not produced on the studied surface. The Clausius-Clapeyron plot of the OH stretching band gave the value of -82.4 kJ/mol as ∆H, the energy of adsorption at the acidic site.
Introduction The acidic properties of zeolites are an important factor in various catalytic reactions and have been investigated by many groups and various techniques.1-10 It is now widely recognized that the acid-catalyzed reactions start by the hydrogen bonding of reactants at the acidic OH groups, the OH groups located between the Al and Si atoms of the framework with strong Brønsted acidity, and that the proton transfer from the OH groups to the hydrogen-bonded adsorbate is a key step.11 A sort of contradiction subsists about the nature of the protonated species, that is, whether the species are stable or only transient on the surface. As summarized in the recently published paper by Haase and Sauer,12 the subject has been investigated by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopies and theoretical calculations have been made to get clues on the question. The paper by Haase and Sauer described the calculation carried out by a HartreeFock type ab initio method for the systems of water or methanol adsorbed on zeolite, and the calculated energy suggested that the oxonium species are in a transient state. This picture agrees with the one derived by Gale et al. who applied density functional theory (DFT) calculation to the adsorption of methanol and methyl cyanide on a zeolite surface.13 Haase and Sauer also calculated the chemical shift of the hydroxyl protons under the two types of adsorption, and the experimental values of chemical shift were just between the values calculated for the neutral and ionized adsorbates leading the authors to suggest the occurrence of an equilibrium between the proton transfer and hydrogen bonding in a real system.12 * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)03127-3 CCC: $12.00
The FT-IR spectroscopy is widely used for the investigation of the structure and environment of surface adsorbates, and the spectra of zeolites with various adsorbates have been measured to derive such information. The FT-IR spectra of water and/or methanol on protonated ZSM-5 zeolite (HZSM-5) were studied to solve the above-mentioned problem of hydrogen bonding/ proton transfer.14-17 The spectra of the systems are characterized by the appearance of broad bands at 3500-3300, ∼2900, ∼2400, and ∼1600 cm-1 which undoubtedly are associated with the adsorption at Brønsted acid OH sites. Until recently, the presence of protonation-produced oxonium ions [HOH2]+ and [CH3OH2]+ was postulated by assigning the 2900 and the 2400 cm-1 bands to the antisymmetric (νa(OH)) and symmetric (νs(OH)) OH stretching modes, respectively. The postulate was not fully convincing, however, since a similar feature is known to appear on the spectra of acetone/HZSM-518-20 and dimethyl ether (DME)/HZSM-521 systems, although the protonation products are of the form [R2OH]+ which has only one ν(OH) mode. Actually, the ∼2900, ∼2400, and ∼1600 cm-1 bands, often called as the (A, B, C) trio, are observed on the vibrational spectra of various adsorbate-covered zeolites. Recently Pelmenschikov et al. proposed a novel intuition22-24 which derives from the scheme used to explain similar feature in liquid phase (see ref 25 for example) and places the origin of the (A, B, C) trio to the neutral species hydrogen-bonded to the acidic OH groups to explain the vibrational spectrum of solid phosphates. They considered that the (A, B, C) trio resulted from the interference by Fermi resonance among the OH stretching and the HOH and HOSi (or HOAl) bending modes which are all broadened by combining with lowfrequency modes such as adsorbate‚‚‚HO stretching mode and overlap together. The interference gives rise to absorption windows inside the overlapped profile and apparent peaks or © 1996 American Chemical Society
11650 J. Phys. Chem., Vol. 100, No. 28, 1996 pseudo-peaks are formed midway between adjacent absorption windows. This picture, which obviously works better in explaining the appearance of the (A, B, C) trio on the vibrational spectra of many zeolite surfaces, does not require the stable existence of ionized species. We have been investigating by FT-IR spectroscopy the adsorption and reaction of methanol on HZSM-5 in order to trace in detail the change of adsorbed methanol to adsorbed DME.26 The system has been studied by Forester and Howe21 who reported on the FT-IR spectrum of the systems prepared at and above room temperature. The adsorption was done by injecting 1 mL of DME gas or 1 µL of liquid methanol into the sample cell. They observed that the injection of DME at room temperature caused the bare surface ν(OH) bands at 3610 cm-1 and 3750 cm-1 to decrease and a broad band to appear at 3300 cm-1. The newly observed band was assigned to the overlap of the ν(OH) band of Brønsted acidic OH groups and that of neutral OH groups, both groups being hydrogen-bonded to DME. They noted that the bands at 2400 and 1600 cm-1 became prominent on raising the injection temperature to 373 K. Referring to the observation that the absorption bands of the reaction product appear at 470 K, the bands were assigned to the ν(OH) band of dimethyloxonium species. We noted, however, that the behavior of the 3300 cm-1 band was different from that of the (A, B, C) trio when a lower amount of DME was introduced into the cell at low temperature and that the picture by Forester and Howe needed further inspection by a detailed experiment. This paper is concerned with the results and interpretation of the FT-IR spectrum of DME/HZSM-5 system observed under various coverage and temperature. The experiment unveiled that the band at 3300 cm-1 was correlated only with the ν(OH) band of the nonacidic silanol OH groups and the (A, B, C) trio to the band of the Brønsted acidic OH group. The adsorption at the acidic OH group was reversible up to 470 K and a value of -82.4 kJ/mol was derived as the heat of adsorption, ∆H. DFT calculation was carried out to estimate the stabilization energy and the normal frequencies of the relevant vibrational modes. The calculation indicated that the stabilization of DME at acidic OH group is due to the hydrogen bonding but the appearance of the (A, B, C) trio cannot be explained unless we incorporate the scheme proposed by Pelmenschikov et al.22-24 Experimental Section FT-IR Experiment. The in situ observation of vibration spectra was carried out at a resolution of 2 cm-1 and at a typical averaging of 64 scans using a Jasco FT-IR 7000 spectrometer equipped with an MCT detector. A HZSM-5 sample with the Si/Al atomic ratio of 27 was provided by Asahi Chemical Industry Co. Ltd. The zeolite powder was pressed into a selfsupporting disk with a typical density of ca. 10 mg/cm2 and placed inside an in situ IR cell. The IR cell with CaF2 windows was attached to a conventional closed gas circulation system. The procedure of sample pretreatment was basically the same as described in ref 27; the sample cell was evacuated by an oil diffusion pump for several hours at 773 K and then at 873 K for 30 min. The sample disk was next exposed at 873 K to 12 kPa of oxygen for 1 h. Partially deuterated zeolite was prepared by exposing at room temperature to 1.3 kPa of D2O gas for 30 min and evacuating at 573 K for another 30 min. The procedure converted 40% of acidic OH groups to OD groups while keeping the neutral silanol groups unchanged. Cleanliness of the thus treated disk was confirmed by an FT-IR measurement. Commercially purchased DME gas of 99.9% purity was passed
Fujino et al.
Figure 1. FT-IR spectra of HZSM-5 interacting with various amount of DME at 173 K. The spectra observed for the sample disk dosed by 0, 540, 600, and 650 Pa of DME are shown in parts a, b, c, and d, respectively. The spectra shown in (b)-(d) were measured after the ambient pressure reached equilibrium value of 0.93 Pa. Absorption band of atmospheric CO2 has been subtracted.
slowly through a dry ice/methanol trap to remove residual impurities before use. DFT Calculation. Ab initio calculation by density functional theory (DFT) based on local density approximation (LDA) was carried out using the DGauss program developed by Cray UniChem Project. The used double-zeta Gaussian basis sets with a polarization function (DZVP) were optimized to minimize the basis set superposition error (BSSE). The acidic hydroxyl group at the bridging site of a zeolite, -Si-(OH)-Al-, was modeled by an H3Si-(OH)-AlH3 cluster. The structure of the cluster was optimized under the constraint that forbids the internal rotation of SiH3 and AlH3 moieties. The potential energy of proton migration from the model cluster to adsorbate DME was then calculated for various values of the distance between the cluster H atom and the O atom of the adsorbate. The calculation was also made of the normal frequencies of the ν(OH) band of the OH group and the ν(CH) bands of the CH3 groups of DME. Results and Discussion IR Bands Observed for the DME on HZSM-5 System. Figure 1 shows the FT-IR spectra of the HZSM-5 disk after adsorption of various amounts of DME at 173 K. Figure 1a is the spectrum observed on bare HZSM-5, and parts b, c, d of Figure 1 are the spectra observed after the introduction of 540, 600, and 650 Pa of DME, respectively. The spectra in Figure 1b-d were measured after the adsorption equilibrium at the ambient pressure of 0.9 Pa was reached. The absorption by atmospheric CO2 has been subtracted in the shown spectra. The peaks at 3610 and 3750 cm-1 are the ν(OH) bands of the acidic OH groups at -Si-OH-Al- bridging sites and the neutral OH groups located at terminal silanol (>Si-OH) groups, respectively. One notes that only the 3610 cm-1 band changes its intensity at a lower dosage of DME giving rise to the broad absorption bands peaking at around 2900 and 2400 cm-1. The region of the 1600 cm-1 band was not accessible because of low transmittance of CaF2 windows. The difference spectra,
Interaction between Dimethyl Ether and HZSM-5 Zeolite
Figure 2. Difference FT-IR spectrum of partially deuterated HZSM5. About 40% of acidic OH groups was replaced by OD while the neutral OH groups remained unchanged. Observation was made at 133 K, and the dosing of DME was made so that the equilibrium pressure was 11 Pa.
Figure 3. Correlation between the 3300 cm-1 band and the ν(OH) band of neutral OH groups. The integrated peak area of the 3300 cm-1 band is plotted against that of the ν(OH) band for the spectra observed at 133 K.
that is, the spectra obtained by subtracting the spectra of the DME/HZSM-5 system, proved the presence of the 2900 cm-1 band underlying beneath the structured peaks originating from the ν(CH) modes of adsorbed DME. The change of the 3750 cm-1 band, which was accompanied by the appearance of the broad band at 3300 cm-1 band, started after the 3610 cm-1 band disappeared. It is also mentioned that a slight change was noticed of the feature of the ν(CH) region at higher dosage of DME presumably due to the onset of physisorption. The above-mentioned correlation between the 3300 cm-1 band and the 3750 cm-1 band is clearly seen by the plots shown in Figure 3 where the integrated absorbance of the 3300 cm-1 band is plotted against that of the 3750 cm-1 band. The correlation was verified by the experiment using partially deuterated zeolite in which 40% of its acidic OH groups were replaced by OD groups but the neutral OH groups ramained unchanged. The zeolite disk was cooled to 133 K and such an amount of DME was dosed that the equilibrium pressure was 11 Pa instead of 0.9 Pa taken in the previous experiment. The difference spectrum derived by dividing the observed spectrum by the bare surface spectrum is shown in Figure 2. Negative peaks are seen at 3750, 3610, and 2668 cm-1 which is the OD version of the 3610 cm-1 band, implying that both the neutral and acidic hydroxyl groups were interacting with DME. The OD band which corresponds to the 3750 cm-1 band is missing at the expected frequency of 2770 cm-1 as the prepared sample lacked the neutral OD groups. Broad bands seen in Figure 1 are all present, but the OD version of the 3300 cm-1 band is missing at the expected frequency of 2300 cm-1. The failure to produce
J. Phys. Chem., Vol. 100, No. 28, 1996 11651
Figure 4. Spectra observed before (a) and after (b) the evacuation of sample cell. Temperature of the HZSM-5 sample was 473 K.
the 2300 cm-1 band despite the appearance of a negative peak at 2668 cm-1 proves that the 2300 cm-1 band is not associated with the acidic OD group and accordingly the 3300 cm-1 band not with the 3610 cm-1 band of normal HZSM-5. We now turn to the IR bands associated with the adsorption at the acidic OH. As seen in Figure 1a-d, the intensity of the ν(OH) band located at 3610 cm-1 on bare surface started to decrease and the (A, B, C) trio along with rather sharp bands of the adsorbed DME appeared as soon as DME gas was introduced. The appearance of the trio preceded the intensity decrease of the silanol OH band and it is straightforward to ascribe the origin of the (A, B, C) trio to the interaction of DME and the acidic OH groups. As mentioned before, the presence of the broad 2900 cm-1 band was obscured by the CH stretching bands until identified by difference spectra. As it turned out, the adsorption of DME onto the acidic OH group was virtually irreversible below room temperature; only the 3750 cm-1 band recovered on evacuating the sample cell below room temperature and the 3610 cm-1 band was recovered by evacuation at higher temperature. No chemical reactions such as the formation of hydrocarbons occurred in the temperature range from 373 to 473 K. A typical example is shown in Figure 4 where the FT-IR spectra observed before and after the evacuation of the cell under the ambient pressure of 160 Pa of DME at 473 K are depicted. It is seen that the evacuation caused the intensity of the 2400 and 2900 cm-1 bands to decrease and that of the 3610 cm-1 band to recover reconfirming that the 2400 and 2900 cm-1 bands are surely associated with the adsorption on the acidic OH groups. Incidentally, the silanol-related 3300 cm-1 band did not appear and the integrated absorbance of the 3750 cm-1 band stayed constant when the adsorption was performed above 373 K. The reversibility of the adsorption enabled us to derive the enthalpy of adsorption, ∆H, by associating the temperaturedependent change of spectral intensity with the ClausiusClapeyron relation. Figure 5 is the Clausius-Clapeyron plot of the data observed in the coverage range of 0.7-0.85 where the coverage of DME on acidic OH sites was determined from the integrated absorbance of the ν(OH) band. The least-squares fit of the plots to the Clausius-Clapeyron equation gave the value of -82.4 kJ/mol for the ∆H. The stabilization energy obtained by the DFT calculation described later was 96.7-89.4 kJ/mol. DFT Calculation of Optimized Structure and Energy of Adsorption. As summarized in the recent review article by Sauer et al.,28 the calculations based on DFT29-31 proved to be quite successful in the interpretation of FT-IR spectra of the
11652 J. Phys. Chem., Vol. 100, No. 28, 1996
Fujino et al. system. And we focus on the shifts of the frequencies, not on the absolute frequencies themselves, caused by the interaction with DME. The calculation gave as optimized structure of free cluster as depicted in Figure 6a. The calculated normal frequency of the ν(OH) mode was 3703 cm-1. The calculation was extended to the systems in which a DME molecule was placed above the OH group of the optimized cluster frame. The energy calculation was performed for two conformers having the orientation of the DME molecule depicted in Figure 6, b and c, that is, the conformers with the COC plane oriented parallel and perpendicular to the SiOAl plane. The calculation which started with the initial conformation of former type changed to the conformation to the latter conformation as optimization progressed. Calculated geometry parameters and stabilization energy for the DME adsorption conformers depicted in Figure 6 are summarized in Table 1. The conformers were stabilized when the two moieties were so located that the hydroxyl H atom was 0.14 nm away from the O atom of DME and the stabilization energy for the conformers depicted by Figure 6, b and c, was 97 and 89 kJ/mol, respectively. The value of 0.14 nm is much longer than the typical length of the OH bond. The possibility of the proton transfer from the zeolite cluster to DME was also examined. When the proton was moved toward the O atom while keeping the distance of the dimethylic O atom from the hydroxyl O atom constant, the potential energy of the system increased, implying that the stabilization originates primarily from the hydrogen bonding. The normal frequencies of the ν(OH) mode were calculated as 2398 and 2580 cm-1 for the conformers shown in Figure 6, b and c, respectively, both being red-shifted by more than 1000 cm-1 from the free cluster value. However, there is no other normal mode whose frequency is close to the peak frequencies of the (A, B. C) trio suggesting that they actually are the pseudo-peaks as postulated by Pelmenschikov et al. The assignment of the 3300 cm-1 band to the OH stretching mode of silanol OH groups under interaction with DME was also tested by the DFT calculation. In the calculation, we adopted two model clusters for our purpose: H3SiOH (a) and (OH)3SiOH (b). The values of 3749 and 3740 cm-1 were derived as the normal frequency of the ν(OH) mode of the isolated clusters (a) and (b), respectively. The optimized structures of the systems consisting of a DME molecule and the model cluster (a) and (b) had the OH stretching frequencies of 3290 and 3166 cm-1 with the frequency shifts of 459 and 574 cm-1, respectively. The values agree nicely with the experiment and support our conclusion that the 3300 cm-1 band originated from the silanol groups which were hydrogen-bonded with DME. Comparison is of observed and calculated frequencies of the OH stretching mode are summarized in Table 2.
Figure 5. Clausius-Clapeyron plots for the reversible adsorption of DME on acidic OH group. The integrated peak areas of the ν(OH) band observed at the coverage ranging from 0.7 to 0.85 are plotted. The variation of the coverage was attained by changing the temperature by the step of 15 K. The least-squares fit gave the value of -82.4 kJ/mol as ∆H of adsorption.
Figure 6. Sketch of the molecular configurations adopted in the DFT calculation. (a) H3Si-OH-AlH3 cluster taken as the model for acidic OH groups, (b) and (c) complexes differing in mutual orientation of DME and the model cluster.
van der Waals complexes formed on solid surfaces. We tested the practicability of the DFT calculation to our experimental results. In the calculation, the acidic OH group of zeolite was modeled by adopting H3Si-OH-AlH3 cluster which was successful in explaining the features of the ν(OH) band under interaction with CO, N2, and rare gas atoms.32 As the molecules such as DME has only one site to interact with zeolite surface, we consider this cluster represents the adsorption feature of this
TABLE 1: Calculated Geometry and Parameters and Adsorption Energies for the Complexes of DME on Various Clustersa geometry parameters Si-Oz Al-Oz Hz-Oz Hz‚‚‚Od Od-Cd Od-Ha Od-Hb Od-Hc Si-Oz-Al Si-Oz-Hz Eads (kJ/mol)
6ab 0.171 0.199 0.098 (0.140)c (0.112)c (0.112)c (0.111)c 118 118
6bb
6cb
(CH3)2O‚‚‚HO-SiH3
(CH3)2O‚‚‚HO-Si(OH)3
0.169 0.194 0.104 0.146 0.142 0.111 0.111 0.110 121 118 89.6
0.169 0.194 0.106 0.144 0.143 0.111 0.111 0.111 123 119 96.9
0.165
0.162
0.100 0.167 0.141 0.111 0.111 0.111
0.101 0.163 1.409 1.114 1.110 1.104
116 50.9
116.0 50.0
Bond lengths and bond angles are given in nm and deg, respectively. Z ) zeolite, D ) DME. b The molecules depicted in Figure 6. c The values in parentheses are the bond lengths of free DME molecule. a
Interaction between Dimethyl Ether and HZSM-5 Zeolite TABLE 2: Observed and Calculated OH Stretching Modes (cm-1) molecule
obs/cm-1
CH3OCH3/SiOH CH3OCH3/Si(OH)Al 6b 6c CH3OCH3/H3SiOH CH3OCH3/(OH)3Si(OH)
3300 2900-1300
calc/cm-1
2398 2580 3290 3166
Conclusion The adsorption of DME on HZSM-5 was investigated by in situ FT-IR spectroscopy and DFT calculations, and the following are the major findings: (1) the DME molecule interacts with both the neutral OH group at silanol site and the acidic OH group at the -Si-(OH)-Al- bridging site; (2) the stronger interaction with the ∆H value of -82.4 kJ/mol takes place at the bridging site; (3) the ν(OH) band of the neutral OH group shifted from 3750 to 3300 cm-1 with broadening on the interaction; (4) the ν(OH) band of the acidic OH groups, on the other hand, gets overlapped and mixed up with overtone and combination modes on interaction with DME, and apparent peaks appear at ca. 2900, 2400, and 1600 cm-1; and (5) the calculation by DFT technique suggested that the origin of the interaction is hydrogen bonding and that the proton transfer from acidic OH group to DME is unlikely. Acknowledgment. This work was supported in part by Grant-in-Aid for Scientific Research (Nos. 06640760 and 07242270) from the Ministry of Education, Science, Sports and Culture of Japan. The authors are grateful to Asahi Chemical Industry Co. Ltd. for providing the HZSM-5 sample. References and Notes (1) Rabo, J. A.; Gajda, G. J. Catal. ReV.-Sci. Eng. 1989-90, 31, 385. (2) Karge, H. G. Stud. Surf. Sci. Catal. 1991, 65, 133. (3) Auroux, A.; Bolis, V.; Wierzchowski, P.; Gravelle, P. C.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2544. (4) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. Stud. Surf. Sci. Catal. 1989, 51. (5) (a) Karge, H. G.; Dondur, V. J. Phys. Chem. 1990, 94, 765. (b) Karge, H. G.; Dondur, V.; Weitkamp, J. J. Phys. Chem. 1991, 95, 283 (6) (a) Borade, R.; Sayari, A.; Adnot, A.; Kaliaguine, S. J. Phys. Chem. 1990, 94, 5989. (b) Borade, R. B.; Adnot, A.; Kaliaguine, S. J. Chem. Soc.,
J. Phys. Chem., Vol. 100, No. 28, 1996 11653 Faraday Trans. 1990, 86, 3949. (7) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; pp 118-284. (8) (a) Thomas, J. M.; Klinowski, J. AdV. Catal. 1985, 33, 199. (b) Klinowski, J. Chem. ReV. 1991, 91, 1459. (9) Pfeifer, H. Colloids Surf. 1989, 36, 169. (10) (a) Sauer, J. Chem. ReV. 1989, 89, 199. (b) Sauer, J. Stud. Surf. Sci. Catal. 1990, 52, 73. (11) Biaglow, A. I.; Gorte, R. J.; Kokotailo, G. T.; White, D. J. Catal. 1994, 148, 779. (12) Hasse, F.; Sauer, J. J. Phys. Chem. 1994, 98, 3083. (13) Gale, J. D.; Catlow, C. R. A.; Carruthers, J. R. Chem. Phys. Lett. 1993, 216, 155. (14) Jentys, A.; Warecka, G.; Lercher, J. A. J. Mol. Catal. 1989, 51, 309. (15) Parker, L. M.; Bibby, D. M.; Burns, G. R. Zeolites 1993, 13, 107. (16) Kubelkova´, L.; Nova´kova´, J.; Nedomova´, K. J. Catal. 1990, 124, 441. (17) Mirth, G.; Lercher, J. A.; Anderson, M. W.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3039. (18) Kubelkova´, L.; Cejka, J.; Nova´kova´, J. Zeolites 1991, 11, 48. (19) Kubelkova´, L.; Nova´kova´, J. Zeolites 1991, 11, 882. (20) Florian, J.; Kubelkova´. L. J. Phys. Chem. 1994, 98, 8734. (21) Forester, T. R.; Howe, F. R. J. Am. Chem. Soc. 1987, 109, 5076. (22) Pelmenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (23) Pelmenschikov, A. G.; van Santen, R. A.; Ja¨nchen, J.; Meijer, E. J. Phys. Chem. 1993, 97, 11071. (24) Pelmenschikov, A. G.; van Wolput, J. H. M. C.; Ja¨nchen, J.; van Santen R. A. J. Phys. Chem. 1995, 99, 3612. (25) Clark, R. J. H.; Hester R. E., Eds. AdVances in Infrared and Raman Spectroscopy; Hiden & Son Ltd.: London, 1980; Vol. 5. (26) Wakabayashi, F.; Kashitani, M.; Fujino, T.; Kondo, J. N.; Domen, K.; Hirose, C. Stud. Surf. Sci. Catal., in press. (27) Wakabayashi, F.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1995, 99, 10573. (28) Sauer, J.; Ugliengo, P.; Garron, E.; Saunders, V. R. Chem. ReV. 1994, 94, 2095. (29) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (30) Trickey, S. B., Ed. AdVances in Quantum Chemistry-Density Functional Theory of Many-Fermion Systems; Academic Press, Inc.: San Diego, CA, 1990; Vol. 21. (31) Labanowski, J. K., Andzelm, J. W., Eds. Density Functional Methods in Chemistry; Springer Verlang Inc.: New York, 1991. (32) Fujino, T.; Furuki, M.; Kashitani, M.; Onda, K.; Kubota, J.; Kondo, J. N.; Wada, A.; Domen, K.; Hirose, C.; Wakabayashi, F.; Ishida, M.; Goto, F.; Kano, S. S. J. Chem. Phys., in press. (33) Ishida, M.; Goto, F.; Wakabayashi, F.; Domen, K. To be submitted for publication.
JP953127X
