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J. Phys. Chem. 1996, 100, 9416-9420
Redox Behavior of CuZSM-5 Catalysts: FTIR Investigations of Reactions of Adsorbed NO and CO H.-J. Jang,† W. Keith Hall,‡ and Julie L. d’Itri*,†,‡ Departments of Chemical Engineering and Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 ReceiVed: August 28, 1995; In Final Form: December 8, 1995X
The redox behavior of CuZSM-5 was investigated by flow microbalance and FTIR measurements using CO and NO as probe molecules. The weight change resulting from switching the flow of 10% O2 in He to pure He corresponded to the removal of extralattice oxygen and approximately 20% autoreduction of total copper ion by one electron. FTIR absorption spectra of CO over CuZSM-5 showed that the concentration of the Cu+ sites changed reversibly upon switching the pretreatment gas from He to O2. The combined experiments (flow microbalance and CO adsorption) proved that CuZSM-5 has a redox function at 500 °C. Continuous measurement of NO adsorption spectra using the same wafer, following a pretreatment in flowing He at 500 °C, also demonstrated the redox properties of CuZSM-5. During the early stage of NO adsorption, a sharp band at 2295 cm-1 appeared that could be assigned in several different ways. Changes occurring as NO underwent reaction on the surface were recorded.
Introduction Over-exchanged CuZSM-5 is one of the most efficient catalysts for decomposition of NO.1-4 Many studies of CuZSM-5 have been made by ESR,4-9 IR,9-18 XANES,19,20 and photoluminescence,7,21 but the chemistry involved upon treatment with various gases, especially those involving oxidation and reduction, is still a subject of debate. Several proposals have been made regarding the nature of the active site for NO decomposition and redox behavior. A number of researchers have suggested that extralattice oxygen (ELO), e.g., the bridged [Cu-O-Cu]2+, is the active species.3,4,14,16,19,22-24 This extralattice oxygen could be removed reductively either spontaneously in Vacuo or by heat treatment with CO or He at elevated temperature.4,10,14 On the basis of in situ ESR experiments, Kucherov et al.5,6 concluded that Cu2+ species do not undergo autoreduction on under-exchanged CuZSM-5. They suggested that these ions are neither reduced nor oxidized by simple thermal treatment and O2 adsorption. Recently, Larsen et al.25 proposed that Cu2+ is autoreduced by thermal treatment with condensation of the hydroxyl radicals [Cu2+OH-]+ to produce Cu+, ESR silent Cu2+O-, and H2O. The authors suggested that both species (Cu2+O- and Cu+) may be involved in the mechanism of NO decomposition. Moreover, they found that the autoreduction process was reversible in the presence of water. The oxygen desorption behavior of a catalyst is an important factor for effective catalysis because surface oxygen produced during NO decomposition inhibits the catalytic function4 and must be removed to regenerate the catalyst. In this study, the redox behavior of CuZSM-5 has been investigated by both flow microbalance and FTIR experiments using CO and NO as probe molecules. The question of spontaneous O2 desorption has been addressed directly. The assignment of some unidentified bands observed during NO adsorption is discussed. As shown by IR experiments using CO,14,15,17,23 spectra from strongly adsorbed CO on Cu+ can be used to follow the increase or decrease in concentration of this species upon pretreatment with various gases. The spectra from * Author to whom all correspondence should be addressed. † Department of Chemical Engineering. ‡ Department of Chemistry. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)02525-1 CCC: $12.00
NO also reveal the presence of Cu+, Cu2+, and Cu2+O-, but this molecule undergoes redox reactions with the catalyst; hence, it must be used judicially in studies of the effects of pretreatment. Experimental Section Materials. Three catalysts were used in this work. They were prepared by the conventional cation exchange method. The degree of copper exchange was 30, 67, and 114% with Si/Al ratios of 14, 25, and 14, respectively. The catalysts were named by cation form, zeolite type, Si/Al ratio, and percentage exchanged, e.g., CuZSM-5-14-114. This catalyst (CuZSM-514-114) was furnished by Air Products and Chemicals Company and used in our earlier work.4 CuZSM-5-25-67 was an aliquot of the catalyst used by the Ford workers5,6 and upon which they performed their EPR experiments. CuZSM-5-11-30 was prepared from NaZSM-5-11-100 supplied by Air Products. The latter two catalysts contained about the same number of copper ions per gram. The gases (1% NO and 4% CO in He, pure He, and O2) were obtained from Matheson, and when necessary suitable absorbents were used to remove trace amounts of water or O2 from the gases. IR Spectra. Spectra were collected by using a Cygnus Model 100 Mattson FTIR spectrometer using pressed wafers of thickness 10-15 mg/cm2. The catalysts were pretreated at 500 °C for 3 h in flowing He or an O2 stream (50 mL/min), unless otherwise noted. IR spectra were recorded at room temperature after exposure of the pretreated catalyst to CO or NO using an IR cell made of quartz and equipped with NaBr windows. The spectrum of an adsorbed species was obtained by subtracting a spectrum recorded in the absence of the probe gas from a spectrum recorded after adsorption of the gas. The adsorption experiments were carried out by first flowing the gas containing the probe molecule at 10-20 mL/min at room temperature and then stopping the flow of gas during time period (up to 20 min) in which spectra were collected. The experiment was sometimes continued by initiating the flow for a second or third time. The probe gases used were 4% CO or 1% NO in He and 1% labeled 15N18O in He. By following changes in the spectra during a sequence of experiments, it was possible to monitor the surface reactions occurring at room temperature. For a given series of experiments, the same wafer was used after each pretreatment at 500 °C. © 1996 American Chemical Society
Redox Behavior of CuZSM-5 Catalysts
Figure 1. Change of catalyst weight upon interchanging 10% O2 in He with pure He at 500 °C: (A) CuZSM-5-14-114; (B) CuZSM-525-67; and (C) CuZSM-5-11-30. Lined-out weights are represented by horizontal bars. The gas flowing over the catalyst is identified above these bars, and the numbers under the bars represent time in hours used to reach the invariant state.
Microbalance Experiment. A Cahn microbalance (Model RG-2000) was used in the flow mode to determine weight changes during adsorption and desorption of O2. Approximately 200 mg of sample was placed in a quartz bucket. Inert gas (He) was flow through the sample with the temperature maintained at 500 °C until a constant weight was achieved. Then a flow of 10% O2 in He was introduced. The associated weight changes obtained upon switching the gas flow between He and 10% O2 in He were determined in a series of experiments. In this way, two fairly reproducible states of system were determined. The typical absolute uncertainty for these weight measurements was e20 µg. By knowing the Cu loading for each sample, weight changes for the lined-out samples could be expressed in units of number of oxygen atoms per Cu atom (O/Cu). Results Flow Microbalance Investigations. The weight changes resulting from switching the gas flow between 10% O2 in He to pure He in repeated cycles were recorded at 500 °C. Figure 1 shows these changes in redox cycles for CuZSM-5-14-114, CuZSM-5-25-67, and CuZSM-5-11-30. These data were recorded after reaching final steady state values in flowing He or in a 10% O2 in He gas stream. As shown in Figure 1, the reproducibility in the several cycles was better for the catalyst with the highest copper loading (CuZSM-5-14-114), but was satisfactory after the first cycle for the other two. The arrows on the right show the average change upon oxidation-reduction calculated as a gain or loss of oxygen from the catalyst ratioed to its Cu content. Interestingly, these are essentially the same for all three catalysts. Thus, the mean values of the O/Cu ratio were 0.1 on CuZSM-5-14-114 and CuZSM-5-11-30 and 0.09 on CuZSM-5-25-67. These data suggest that approximately 20% of the Cu2+ must have been reduced in Cu+, assuming a two electron transfer process. FTIR Spectra of Adsorbed CO. CO adsorption on CuZSM5-14-114 was investigated by FTIR. CO is adsorbed exceptionally strongly on Cu+ sites, but only weakly on Cu2+ at room temperature.7,14,15,17 The adsorption spectra of CO on samples pretreated in He and pretreated in pure O2 are shown in panels A and B of Figure 2, respectively. After the first exposure to flowing 4% CO in He, a band was observed at 2157 cm-1 along
J. Phys. Chem., Vol. 100, No. 22, 1996 9417
Figure 2. IR spectra of CO adsorption on CuZSM-5-14-114 pretreated with He (A) and pretreated with O2 (B). Spectra a, b, and c in both panels A and B were obtained after 1, 8, and 18 min static (30 Torr), respectively, after admission of 4% CO in He, and spectra d, e, and f were obtained after 1, 8, and 18 min static, respectively, after the second admission of 4% CO in He for 2 min. CO adsorption intensities on the catalyst pretreated with He were over 2 times greater than those on the catalyst pretreated with O2 (see calibration bars). Interestingly, the Cu+ sites were not completely eliminated by the O2 pretreatment.
with a tiny shoulder band at 2110 cm-1. The band intensity increased during the equilibration period (Figure 2A, a-c). The position of the band at 2157 cm-1 shifted to 2154-2151 cm-1 with the simultaneous appearance of another band at 2177 cm-1 at higher CO concentration (Figure 2A, d-f). During this equilibrium period, the intensity of CO absorption bands increased slightly, but the system reached an equilibrium after 18 min in the static mode. These spectra are entirely consistent with those published by Anpo et al.,7 who assigned the band at 2157 cm-1 as Cu+-CO and bands at 2151 and 2177 cm-1 as antisymmetric and symmetric stretching modes of Cu+(CO)2, respectively. For CuZSM-5-14-114 pretreated in O2, in addition to the 2157 and 2177 cm-1 bands, a higher frequency band appeared at 2213-2203 cm-1 (Figure 2B). This has been assigned previously to CO adsorbed on isolated Cu2+ ions.8,26 This band shifted to the lower frequency in that range, i.e., 2203 cm-1, with higher CO concentration and longer duration in the static mode. The variation in CO absorption spectra on oxidized CuZSM-5-14-114 was similar to that on the catalyst pretreated with He, but all intensities were weaker on the oxidized catalyst, suggesting that Cu+ sites had been converted to Cu2+, whatever species is formed (Cu2+ or Cu2+O2-). Apparently, the Cu+ cannot be completely oxidized at 500 °C in pure O2, even upon cooling slowly in O2 to room temperature. The reversibility of the CO absorption intensities was observed by changing the pretreatment gas between O2 and He in continuous cycles (Figure 3A), similar to those obtained in the flow microbalance investigation. The same wafer was used during the entire experiment, and each spectrum was obtained under the same experimental conditions after reaching an equilibrium state. Because each spectrum has two strong bands from CO adsorbed on Cu+ sites, two intensities at 2151-2153 and 2177 cm-1 are plotted in Figure 3. Reversible behavior was observed for CO adsorption on CuZSM-5-14-114 for pretreatment in both He and O2, respectively (Figure 3A). The intensities of the CO absorption on the oxidized catalyst were less than 50% of those of CO on the He-pretreated sample. CO adsorption on the oxidized catalyst that was not flushed with He at room temperature before exposure to CO resulted in the
9418 J. Phys. Chem., Vol. 100, No. 22, 1996
Figure 3. Variation of CO absorption band intensity over CuZSM5-14-114 (A) with redox cycles at 500 °C upon switching the pretreatment gas between O2 and He and that over CuZSM-5-25-67 (B) with changing pretreatment. All of the CO absorption spectra were obtained at room temperature after a second admission of 4% CO in He, followed by 18 min static; the equilibrated spectra had two strong bands at 2153-2151 and 2177 cm-1 corresponding to Cu+(CO)2. The intensities obtained following pretreatment are shown separately for the different gases employed indicated at the bottom of the figure.
same spectrum as the oxidized sample, which was purged with He but with lower intensity. Similar variations in intensities were exhibited on the underexchanged CuZSM-5-25-67 (the catalyst used by Kucherov et al.)5,6 with changing pretreatment (Figure 3B). In this case, the catalyst not only exhibited spontaneous reduction but was also reduced with 4% CO in He at 500 °C for 2 h. The intensities of adsorbed CO changed reasonably with the change of the pretreatment gas; the intensity was greatest on the reduced catalyst and smallest on the oxidized catalyst. The band intensities of adsorbed CO on CuZSM-5-25-67 were weaker than those on CuZSM-5-14-114 due to the lower concentration of copper. Thus, the same catalyst upon which Kucherov et al.5,6 did their EPR work contains EPR silent Cu+, even though they reported that all of the copper sites were present as Cu2+. Evidently EPR is not an effective tool for quantifying these effects. IR Spectra of Adsorbed NO. An IR study was conducted by using 1% NO in He in contact with CuZSM-5-14-114, and the spectra of adsorbed NO are shown in Figure 4. The initial exposure to flowing 1% NO in He at room temperature produced only a band at 2295 cm-1 (Figure 4a). As the flow was continued, bands related to NO interacting with copper species appeared, including monomeric NO on Cu2+ at 1907 cm-1 and on Cu+ at 1812 cm-1.9,11,13,18 The dinitrosyl bands on Cu+ did not appear, possibly because of the low partial pressure of NO used. The band at 2295 cm-1 decreased and disappeared between steps (a-d), while the band at 2133 cm-1 appeared and continued to increase. This band has recently been studied by Hoost et al.27 and assigned as NO2+ held on the lattice and associated with the Brønsted sites. The band at 2295 cm-1 may be attributed to adsorbed N2.28,29 Additional bands associated with oxidized NO in the 1600-1300 cm-1 region were present and continued to increase with higher NO exposure (not shown in Figure 4). The band at 1812 cm-1 disappeared while the band at 1905 cm-1 increased, demonstrating the conversion of Cu+ to Cu2+.
Jang et al.
Figure 4. IR spectra of NO adsorbed on pretreated CuZSM-5-14-114 (a) after flowing 1% NO in He for 1 min; (b) after flowing of 1% NO in He for an additional minute; (c) after 9 min in static condition following (b); (d) after flowing 1% NO in He for another 10 s following (c); (e) after flowing of 1% NO in He for 10 s following (d) plus 9 min static. The Experimental Section describes this technique in more detail.
Figure 5. IR spectra of NO adsorbed on the CuZSM-5-14-114 wafer used in the NO adsorption experiment of Figure 4 after treatment in flowing He at 500 °C, which completely removed all adsorbed species: (a) after first flowing 1% NO in He for 1 min; (b) after continuing the flow for an additional minute; (c) after continuing the flow for 30 s; (d) after a second 30 s followed by 4 min static; (e) after flowing another 30 s followed by 9 min static.
The experiment of Figure 4 was repeated on the same wafer, following a pretreatment in flowing He at 500 °C that removed all bands shown previously. The spectra (Figure 5) obtained upon dosing with NO were similar to those on the fresh catalyst. After the initial exposure to NO, a band at 2295 cm-1 appeared. Bands at 1910 and 1812 cm-1 also grew with exposure to NO, but the latter was attenuated as the experiment continued. The other bands related to oxidized NO species, including NO adsorbed on Cu2+ at 1910 cm-1, increased rapidly, suggesting that the Cu+ sites were being oxidized to Cu2+. The lowfrequency shoulder of this band, centered at about 1895 cm-1, has been reported previously.9 Recently, it was proposed that
Redox Behavior of CuZSM-5 Catalysts
Figure 6. IR spectra of adsorbed labeled 15N18O on CuZSM-5-14114 pretreated in He at room temperature: (a) adsorbed 15N18O; (b) adsorbed NO.
it corresponds to a mononitrosyl adsorbed on the Cu2+ O- ion pair.30 The remaining spectra confirmed the changes reported in Figure 4. IR Spectra of Adsorbed 15N18O. To learn more about the band at 2133 cm-1 observed during NO adsorption, labeled 15N18O was introduced over the CuZSM-5-14-114 pretreated with He. Three distinct bands appeared when unlabeled NO was used (Figure 6b). Those are the bands due to NO adsorbed on Cu2+ and Cu+ at 1908 and 1814 cm-1, respectively, and the broad band at 2133 cm-1. By using 15N18O as a probe molecule, several new bands appeared (Figure 6a). These were doublets shifted from the positions of the original three NO bands observed with 14N16O. The bands at 1875 and 1780 cm-1 resulted from 15N16O adsorbed on Cu2+ and Cu+, respectively. They were also obtained in the spectra from labeled 15N16O (not shown). The bands at 1826 and 1733 cm-1 were also shifted from 1908 and 1814 cm-1 by 15N18O on Cu2+ and Cu+, respectively. These data show that oxygen exchange occurs rapidly between labeled 15N18O and zeolite framework oxygen or ELO during 15N18O adsorption, in agreement with the results of Valyon and Hall.12 Discussion The redox flow microbalance and FTIR experiments demonstrate that oxygen is adsorbed and desorbed reversibly in the presence and absence of gaseous O2, confirming the results of Li and Hall.4 The CuZSM-5 acts as if it has an O2 sublimation pressure, in agreement with the mechanism proposed by these workers. The degree of weight change as a result of oxygen desorption and adsorption was in proportion to the amount of Cu present in the preparation. The O/Cu ratio, representing the degree of reduction (or reoxidation) of copper present in the CuZSM-5, was virtually constant at 0.1, regardless of copper content or the Si/Al ratio. This means that about 20% of total copper ions are “autoreducible” with flushing the catalyst in flowing He at 500 °C, as reported previously.4 However, flow microbalance results show only the oxygen desorption and adsorption behavior of CuZSM-5 resulting from switching the flowing gas between O2 and He. They do not in themselves prove that the sorption-desorption process is a redox process. Whether the desorption and adsorption of oxygen are related to the reduction and oxidation of copper has not been previously investigated by an IR study of CO. CO is known to act as a
J. Phys. Chem., Vol. 100, No. 22, 1996 9419 probe molecule diagnostic for the Cu+ sites on CuZSM5.8,14,15,17,31-33 Interaction of the dipole moment of CO molecules with centers having a net positive charge increases the C-O stretching frequency from that of the free molecule (2143 cm-1). The variation in the CO absorption band intensity on CuZSM-5-14-114 induced by switching the pretreatment gas between O2 and He (Figure 3) shows that CuZSM-5 undergoes autoreduction. The Cu+ concentration simply decreased upon oxidation and increased after autoreduction by flushing with He at 500 °C. This phenomenon occurred even with CuZSM5-25-67, the same catalyst used by Kucherov et al.5,6 (Figure 3B). With this catalyst, the absolute amount of O2 evolved was much less than that with CuZSM-5-14-114, but the extent of the reduction-oxidation was the same. Indeed, it was very similar regardless of preparation, copper content, and Si/Al ratio. The microbalance results indicate a reversible uptake (loss) of oxygen during exposure to O2 (He) at 500 °C. The FTIR results clearly show a concomitant oxidation (reduction) of Cu ions. Such a reversible oxidation of Cu ions at this temperature is consistent with the models proposed for decomposition of NO. One of the species often suggested as the reducible Cu moiety is an oxocationic Cu species, (Cu2+-O-Cu2+).4,14,31 Decomposition of this bridged oxygen species at high temperature leaves two Cu+ cations associated with the zeolite framework. Our results indicate that there is an increase in the Cu+ concentration during heating in He at 500 °C. Other reducible Cu moieties that have been proposed are Cu2+O- and Cu2+O2-,30 both of which are reduced to Cu+ and, thus, are also consistent with the observed redox chemistry. An interesting fact shown in the CO adsorption spectra was that some of Cu+ remained even after extended treatment with O2. These results were in agreement with a XANES investigation conducted by Liu and Robota.20 They also reported that a small fraction of Cu(I) remained even in a highly oxidizing environment. Interestingly, Li and Hall4 found that the linedout weight of the catalyst in O2/He streams at 500 °C was dependent on the concentration of O2 in the stream. Thus, apparently there will always be Cu+ present in an amount dependent on the ambient atmosphere. The redox behavior of CuZSM-5 was also observed in the FTIR study of NO adsorption at room temperature. Continuous NO spectra (Figures 4 and 5) showed that CuZSM-5 may be oxidized by exposure to NO, but is restored to its initial condition by pretreatment in flowing He at 500 °C. This was demonstrated by the regeneration of the band at 1812 cm-1 and the sharp band at 2295 cm-1. At the first stage of NO adsorption, only a band at 2295 cm-1 was observed, which may have been caused by N2 formed concomitant with oxidation of the catalyst.29,34 Complete conversion of NO was possible with the first NO admission, in which approximately 7-9 µmol of NO was introduced to CuZSM-5-14-114 containing about 10 µmol of copper. The band at 2295 cm-1 did not appear on CuZSM-5 reduced with H2 and was very weak on CuZSM-5 treated with O2. Conceivably, it could be assigned as an overtone of a superoxide.14,35-39 Li and Onishi et al.35 found the fundamental vibration of superoxide with the adsorption of O2 on CeO2 in the 1126 cm-1 region together with the first overtone at 2252 cm-1, where in our work the transmission was essentially zero in this regime due to strong broad lattice vibrations nearby. This vibration was reported by others,29 but, in our opinion, a satisfying explanation of its origin has not been given. During NO adsorption, the other unknown band at 2133 cm-1 was observed, and it continued to increase with higher NO exposure (Figure 4). Many researchers have reported this band;
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Jang et al.
TABLE 1: Observed and Calculated Frequency Shift by the Labeled 15N16O and 15N18O from a Band at 2132-2133 cm-1 on NO Adsorption Spectra N16O (cm-1)
15 +a
NO NO2+ b observed frequencyc
2094-2095 2084.5-2085.5 2090-2094
N18O (cm-1)
15
2037-2038 2043-2044 2037-2043
Assuming that the band is NO+. b Assuming that the band is NO2+. c Observed frequency on the spectra of adsorbed 15N18O. a
it is relatively strong and distinct in the NO adsorption spectra.10,16,27,29 Only recently, however, Hoost et al. concluded that this band was associated with adsorbed NO2+.27 By using 15N18O as a probe molecule, two kinds of bands appeared as shown in Figure 6. One is caused by 15N18O and the other by 15N16O, which is formed by oxygen exchange between 15N18O and the zeolite framework oxygen.12 The band at 2133 cm-1 was shifted to 2090 or 2042 cm-1 by 15N16O or 15N18O, respectively. Calculations after assuming that the band was either NO or NO2 adsorbed on a highly oxidizing site were made and are shown in Table 1. The calculation for the latter was made by using the formula given by Nakamoto40 from the band at 2133 cm-1 observed herein, assuming that the bond angle of NO2 is 134.17°, i.e., that of the gaseous molecule. On the basis of the frequency shift, NO+ appears to fit better than NO2+ in some spectra, whereas NO2+ fits better in the others. Either NO+ or NO2+ could be formed. The relatively broad band at 2133 cm-1 suggests that it was composed of more than one species, although Bell et al.29 and Iwamoto et al.16 have assigned it as NO2+. The most definitive work is the recently published work by Hoost et al.,27 who concluded that it is NO2+ held on the zeolite lattice. Our isotope work (Table 1) is consistent with a diatomic oscillator. Conclusions (1) The microbalance data showed that CuZSM-5 catalysts undergo autoreduction to approximately the same extent regardless of the extent of exchange or Si/Al ratio. Under our conditions the extent amounted to approximately 20% reduction of total copper by one electron. (2) IR adsorption spectra using CO as a probe molecule confirmed that the concentration of Cu+ sites varied in the same way as the redox treatments. (3) IR spectra of NO adsorbed at room temperature are time dependent. The band at 1810 cm-1 demonstrated the presence of Cu+ sites, while that at 1910 cm-1 corresponded to adsorbed NO on Cu2+. The conversion of Cu+ to Cu2+ with increasing exposure to NO was evident. (4) Upon flushing the results of (3) with He at 500 °C, the catalyst underwent autoreduction so that the cycle could be repeated. (5) A sharp band at 2295 cm-1 of unknown origin was observed at room temperature on CuZSM-5 in the early stages of NO adsorption. It has been previously attributed to chemisorbed N228,29 or to a superoxide formed during the NO chemistry. This deserves further study. Acknowledgment. We thank the Ford Motor Company for supplying us with an aliquot of the catalyst used in their work.5,6 We are grateful to the Department of Energy, Basic Energy
Sciences, for providing the financial support for this work (Grant No. DE-FG02-95ER14539). J.L.D. gratefully acknowledges financial support from the Petroleum Research Fund (ACS-PRF No. 28479-G5). References and Notes (1) Iwamoto, M.; Furukawa, H.; Kagawa, S. In New DeVelopment in Zeolite Science and Technology; Murakami, Y., Ed.; Elsevier: New York, 1986; p 943. (2) Iwamoto, M. Stud. Surf. Sci. Catal. 1990, 54, 121. (3) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, 213. (4) Li, Y.; Hall, W. K. J. Catal. 1991, 129, 202-215. (5) Kucherov, A. V.; Gerlock, J. L.; Jen, H. W.; Shelef, M. J. Phys. Chem. 1994, 98 (18), 4892-4894. (6) Kucherov, A. V.; Gerlock, J. L.; Jen, H. W.; Shelef, M. Zeolite 1995, 15, 9-14. (7) Anpo, M.; Matsioka, M.; Shioya, Y.; Yamahita, H.; Giamello, E.; Morterra, C.; Che, M. J. Phys. Chem. 1994, 98, 5744. (8) Amara, M. Appl. Catal. 1987, 35, 153-168. (9) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97 (6), 1204-1212. (10) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 9 (27), 7054-7060. (11) Valyon, J.; Hall, W. K. In New Frontiers in Catalysis: Proceedings of the 10th International Congress on Catalysts; Guczi, L., et al., Eds.; Elsevier Science: Amsterdam, 1992; pp 1339-1350. (12) Valyon, J.; Hall, W. K. J. Catal. 1993, 143, 520-532. (13) Valyon, J.; Hall, W. K. Catal. Lett. 1993, 19, 109-119. (14) Sarkany, J.; d’Itri, J. L.; Sachtler, W. M. H. Catal. Lett. 1992, 16, 241. (15) Sarkany, J.; Sachtler, W. M. H. Zeolites 1994, 14, 7. (16) Iwamoto, M.; Yahiro, H.; Mizuno, N.; Zhang, W. X.; Mine, Y.; Furukawa, H.; Kagawa, S. J. Phys. Chem. 1992, 96 (23), 9360-9366. (17) Spoto, A. Z.; Bordiga, S.; Ricchiardi, G.; Martra, G. Appl. Catal. B: EnVironmental 1994, 3, 151-172. (18) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510-520. (19) Gru¨nert, W.; Hayes, N. W.; Shpiro, J. R. W.; Siddiqui, M. R.; Baeva, G. N. J. Phys. Chem. 1994, 98, 10832-10846. (20) Liu, D. J.; Robota, H. J. Catal. Lett. 1993, 21, 291. (21) Dedecek, J.; Wichterlova, B. J. Phys. Chem. 1994, 98 (22), 57215727. (22) Hall, W. K.; Valyon, J. Catal. Lett. 1992, 15, 311. (23) Lei, G. D.; Adelman, B. J.; Sarkany, J.; Sachtler, W. M. H. Appl. Catal. B: EnVironmental 1995, 5, 245-256. (24) Petunchi, J. O.; Marcelin, G.; Hall, W. K. J. Phys. Chem. 1992, 96 (24), 9969-9975. (25) Larsen, S. C.; Aylor, A.; Bell, A. T.; Reimer, J. A. J. Phys. Chem. 1994, 98, 11533-11540. (26) de Jong, K. P.; Geus, J. W.; Joziasse, J. J. Catal. 1980, 65, 437. (27) Hoost, T. E.; Lafrauboise, K. A.; Otto, K. Catal. Lett. 1995, 33, 105-116. (28) Kuroda, Y.; Konno, S.; Morimoto, K.; Yoshikawa, Y. J. Chem. Soc., Chem. Commun. 1993, 18; J. Phys. Chem. 1995, 99, 10621. (29) Bell, V. A.; Feeley, J. S.; Deeba, M.; Farrauto, R. J. Catal. Lett. 1994, 29, 15. (30) Alyor, A.; Larson, S. C.; Reimer, J. A.; Bell, A. T. J. Catal., in press. (31) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95, 3727. (32) Zeccina, A.; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnell, L. J. Phys. Chem. 1994, 98, 9577-9582. (33) Huang, Y. Y. J. Catal. 1973, 30, 187. (34) Li, Y.; Armor, J. N. Appl. Catal. 1991, 76, L1. (35) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J. Am. Chem. Soc. 1989, 111, 7683-7687. (36) Kasai, P. H. J. Chem. Phys. 1965, 43 (9), 3322-3327. (37) Lunsford, J. H.; Jayne, J. P. J. Chem. Phys. 1966, 44 (4), 14871492. (38) Lunsford, J. H. J. Chem. Phys. 1967, 46 (11), 4347. (39) Fadini, A.; Schnepel, F. M. Vibrational Spectroscopy Methods and Applications; Ellis-Horwood: New York, 1989. (40) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; John Wiley and Sons: New York, 1978; pp 112.
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