ZSM-5 Catalyst and Its

Feb 8, 1994 - Elio Giamello and Claudio Morterra. Dipartimento di Chimica Inorgánica, ... Marye Anne Fox. Department of Chemistry, The University of ...
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J. Phys. Chem. 1994, 98, 5744-5750

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Preparation and Characterization of the Cu+/ZSM-5 Catalyst and Its Reaction with NO under UV Irradiation at 275 K. In Situ Photoluminescence, EPR, and FT-IR Investigations Masakazu Anpo,' Masaya Matsuoka, Yasushi Shioya, and Hiromi Yamashita Department of Applied Chemistry, University of Osaka Prefecture, Gakuen-cho 1 - 1 , Sakai, Osaka 593, Japan Eli0 Giamello and Claudio Morterra Dipartimento di Chimica Inorganica, Chimica Fisica dei Materiali, Universita di Torino, Via Pietro Giuria 9, 101 25 Torino, Italy

Michel Che Laboratoire de Reactiuite de Surface, Universitd P. et M . Curie, UA 1106-CNRS,4 Place Jussieu, Tour 54, 75252 Paris Cedex 05, France

Howard H. Patterson, Steven Webber, and Stephen Ouellette Department of Chemistry, University of Maine, Aubert Hall, Orono, Maine 04469

Marye Anne Fox Department of Chemistry, The University of Texas at Austin, Texas 78712 Received: February 8, 1994"

The characterization of the Cu+/ZSM-5 catalyst (1.9 wt % Cu) prepared from the ion-exchanged Cu2+/ZSM-5 sample by evacuation a t higher temperatures has been undertaken by in situ photoluminescence, EPR, and FT-IR spectroscopy. EPR measurements of the Cu2+ signal indicate that evacuation of the CuZ+/ZSM-5 system a t temperatures higher than 373 K leads to a decrease in the intensity of the EPR signal, suggesting that Cu2+ is chemically reduced to Cu+ by this thermal vacuum treatment. Only the reduced Cu+/ZSM-5 catalysts exhibit photoluminescence spectra a t around 420-550 nm, attributed to the radiative decay from excited Cu+ ions within the ZSM-5. The decrease in the intensity of the E P R signal due to Cu2+ is closely related to the increase in the photoluminescence intensity due to Cu+. The addition of NO onto the Cu+/ZSM-5 catalyst leads to the formation CU+~+-NO&adducts and dynamic quenching of the photoluminescence, suggesting that Cu+ reacts with N O not only in the ground state but also in the excited state. UV irradiation of the Cu+/ZSM-5 catalyst in the presence of NO leads to the photocatalytic decomposition of N O into N2 and 02 a t temperatures as low as 275 K. In situ photoluminescence, EPR, and FT-IR measurements suggests that a local charge separation involving electron transfer from the excited Cu+ ion to the a-antibonding orbital of NO is involved in the decomposition of NO on the catalyst under UV irradiation.

Introduction Reducing global air pollution caused by NO,, as well as C02 and SO,, is currently an urgent and demanding challenge.l-10 Ion-exchanged copper/zeolite catalysts have attracted a great deal of attention as potential catalysts for direct decomposition of NO, into N2 and 02.11-1s On the other hand, we have reported that Cu2+ ions anchored on Si02 surfaces are reduced to Cu+ when the Cu2+/Si02sample is evacuated at temperatures above 573 K. These Cu+/SiO2 catalysts induce the decomposition of N O photocatalytically and stoichiometrically into N2 and 0 2 at 275 K.19-21 These results suggest that partially reduced Cu+ species play a significant role as active species in the catalytic and photocatalytic decomposition of N O into N2 and 0 2 . In recent years, the preparation and the characterization of the supported catalysts such as vanadium and molybdenum oxides and copper-ion-containingcatalysts have been extensively studied and much information has been reported on the chemical state of the active surface sites and its influence upon the catalytic properties. However, the most widely used methods are often not sufficientlysensitive for highly dispersed, supported catalysts with catalyst loadings lower than 0.1-0.3 wt %. As reported in our previous paper,22$23 in situ photoluminescencestudies of these 0

Abstract published in Advance ACS Absrracrs, May 1, 1994.

0022-3654/94/2098-5744%04.50/0

catalysts are useful in the elucidation of the surface structure and the nature of relevant excited states of these highly dispersed catalysts, because of the high sensitivity and nondestructive nature of the photoluminescence method. Cu+ ions supported on Si02,20,21 A1@3,24*25 and zeolite^*^^^^ exhibit photoluminescence around 400-600 nm when they are excited by 300-nm light. It is, therefore, interesting to study the structure and nature of the surface active sites on the copper/ZSM-5 catalyst by in situ photoluminescence with EPR and FT-IR spectroscopy, since it has been suggested that the Cu+ site plays a significant role in the catalytic and photocatalytic direct decomposition of N O on such catalysts.11-21 With well-defined anchored catalysts, one can expect not only to achieve more active and selective photocatalytic systems, but also to obtain more detailed information about the mechanisms of photocatalysis on a molecular scale.28 Furthermore, with this anchoring method one can disperse the active sites and design the active surface, as shown in Cu+/SiO2 catalysts prepared by ionexchange.20.21 It is of special interest to design the ion and/or cluster size catalysts within the zeolites29 because these fascinating supports offer unique nanoscaled pore systems, unusual internal surface topology, and ion-exchangecapacities. The pore structure of the zeolite not only allows for reactant molecules to diffuse into the pore where it can access the catalyst anchored within the 0 1994 American Chemical Society

Characterization of the Cu+/ZSM-5 Catalyst zeolite cavities, but also can remain intact during subsequent ion and/or cluster growth. The catalysts themselves must be either highly dispersed or very small. In fact, the advantages of such shape-selective catalysis within zeolites have been proved in a number of significant industrial processes. In addition, the modification of the space required for a specific photocatalytic reaction is important. Recently, Domen et al.30 found that niobate-mixed oxides with a layered perovskite structure exhibit efficient photocatalytic activity for Hz evolution from HzO. These characteristic photocatalytic properties might be associated with the modified structure of the interlayer spaces. Unique photocatalytic properties which cannot be realized in normal catalytic systems can be expected in modified reaction spaces. Zeolites with well-defined nanopore structure provide one of the most promising modified spaces for photocatalytic reactions. Along these lines, in the present work, the characteristics of Cu+ species anchored in the nanopores of ZSM-5 zeolite and their reaction with gaseous NO at 275 K under UV irradiation are investigated on a molecular scale by in situ photoluminescence, EPR, and FT-IR measurements.

The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5745 A

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Figure 1. EPR spectrum of the Cu2+/ZSM-Ssample evacuated at 373 K. (EPR signal was recorded at 285 K.)

Experimental Section Preparation of the Catalyst. A Cu2+/ZSM-5 sample (Si:Al ratio = 23.3) was prepared by ion-exchange with an aqueous (Cu(NH3)4)z2+solution. After thesample was washed with water and dried in air, its copper loading was determined by an inductively coupled plasma emission spectrometer (Plasma 300 from Allied Analytical System) as 1.9 wt 7'% metal. Apparatus and Procedures. A quartz cell with window and furnace sections, having a volume of ca. 50 cm3, was connected to a vacuum system. Prior to the measurements of spectra and photoreactivity, the samples were pretreated as follows: (a) degassing a t 673 K for 1 h, (b) heating a t the same temperature under 20 Torr 0 2 for 1 h, and (c) evacuation a t the desired temperature. Photoluminescence of the catalyst and the lifetimes were recorded at 77 and 293 K with a Shimadzu RF-501 spectrofluorophotometer and an apparatus for the lifetime measurement (consisting of a Nz laser, a monochromator, lenses, and a Hamamatsu R955 photomultiplier, an Iwatsu TS8123 storage scope, a GP-IP(1EEE-488), and a NEC-PC9801 computer), respectively. Synchronous scan photoluminescence measurements were made on a Perkin-Elmer MPF-44A spectrafluorometer. In these measurements the excitation and emission wavelength drives of the photoluminescence spectrometer were scanned simultaneously with a constant wavelength difference (Ay = 160 nm) between them. EPR spectra were recorded on a JES-RE-2X and a Varian E-109 spectrometer (X-band) at 77 and 295 K. FT-IR spectra were recorded on a Bruker IFS 113v spectrometer at 295 K. After this pretreatment, the catalyst (0.1 g) was spread on the quartz window (surface area ca. 20 cm2). Photoreactions were carried out at 275 K using the output of a Toshiba (SHL-100 UV) mercury lamp passed through a color filter (A > 280 nm) and a water filter. Reaction products were analyzed by gas chromatography and mass spectrometry. Results EPR Studies of the Preparation of the Cu+/ZSM-5 Catalyst. Figure 1 shows the EPR spectrum of a CuZ+/ZSM-5sample that had been evacuated at 373 K. This signal is broad and scarcely resolved and is typical of hydrated Cu*+ ions in the zeolitic framework. Evacuation at higher temperatures led to a change in the EPR spectral profile and to the appearance of various distinct Cuz+ bands. Figure 2 (curve a) shows the effect of the evacuation temperatureon theintensity of theEPRsignal assigned to C d + . The increased evacuation temperature leads to a drastic decrease in the intensity of the signals with very minor changes in their spectrum parameters and line shapes. As shown in Figure 3, the EPR spectrum of a CuZ+/ZSM-5 sample which had been

Degassing Temperature / K

Figure 2. Effects of the evacuation temperature of the Cu2+/ZSM-S sample on the relative intensity of the EPR signal due to Cu2+(a), the relative yields of the photoluminescence due to Cu+ (b), the relative conversions (yields) of the photocatalytic decomposition of NO at 275 K (c), and the amount of CO molecules selectively adsorbed on the Cu+ ions on the catalyst at 285 K (d).

d gi=233 A / = 1 5 0 G

7

-&z-Lz 7

/

x2

M = 2 2 7 A =173G

Figure 3. EPR spectra of the CuZ+/ZSM-5sample evacuated at 773 K. (EPR signals were recorded at 285 K.)

evacuated a t 773 Kconsists of three species with slightly different spin-Hamiltonian parameters (A: 811 = 2.333, All = 150, g, = 2.04. B: 81 = 2.31, All = 157, g, = 2.05. C: gl = 2.27, Ail = 173G,g, = 2.07). These findings are the same as those previously reported for the Cuz+/ZSM-5 after the thermovacuum treatment at higher temperatures.31-34 However, evacuation at temperatures higher than 1073 K led to the disappearance of the signal with gll = 2.27, All = 173 G, and g, = 2.07 ( C species): the signal consists of only two species (A and B) with very weak intensities. In theseevacuation stages, the color of the samples clearly changed from blue to white, indicating that the chemical reduction of Cu2+ to Cu+ had occurred. The thermal reduction of the Cuz+ ions in zeolites under vacuum has previously been reported.33~36 Although the reducibility of the Cu2+ ions was different from that of ZSM-5, a similar profile of the EPR signal assigned to the product of the chemical reduction of Cu2+was also observed for the C ~ ~ + / Y - z e o l i t eand ~ ~ ,Cu2+/Si02.19-z' ~* Photoluminescence Studies of the Cu+/ZSM-5 Catalyst and Its Interaction with NO. As mentioned above (Figure 2), only a very weak EPR signal assigned to Cu2+ could be observed in the CuZ+/ZSM-5samples in which the Cu2+ ions were reduced to the Cu+ ion by evacuation at temperatures above 673 K. With these Cu+/ZSM-5 catalysts, photoluminescence became observable upon excitation around 300 nm. Figure 4 shows a typical photoluminescence spectrum obtained at 77 K of the Cu+/ZSM-5 catalyst (prepared by the evacuation of the Cuz+/ZSM-5 sample at 1173 K) with its corresponding excitation spectrum. These spectra have been attributed to the presence of Cu+;19-21,2G27 Le.,

5146 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994

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300

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Wavelength I nm Figure 4. Observed ordinary photoluminescence spectrum and its deconvoluted spectra (emission X, Y,and Z) and its excitation spectra (EX1,2, and 3) of the Cu+/ZSM-5 catalyst which was prepared by the evacuation of the Cu2+/ZSM-5 sample at 1173 K. (The excitation spectrum was monitored at 410 nm (EXl), 460 nm (EXz), and 510 nm (EX3), respectively. The spectra were recorded at 77 K.) 400

500

400

600

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Wavelength I nm Figure 6. Observed synchronous scan photoluminescence spectrum of the Cu+/ZSM-5 catalyst which was prepared by the evacuation of the Cu2+/ZSM-5 sample at 873 K (measured at 298 K, Ay = 160 nm).

, I

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Time I ~s Figure 7. Decay curves of the photoluminescence spectrum of the Cu+/ ZSM-5 catalyst which was prepared by the evacuation of the Cu2+/ ZSM-5 sample at 1173 K.

Wavelength I nm Figure 5. Observed ordinary photoluminescence spectrum and its deconvoluted spectra (emission X, Y,and Z) and its excitation spectra (EXl, 2, and 3) of the Cu+/ZSM-5 catalyst which was prepared by the evacuation of the Cu2+/ZSM-5 sample at 973 K. (The excitation spectrum was monitored at 410 nm (EXl), 460 nm (EX2), and 510 nm (EX3), respectively. The spectra were recorded at 77 K.) the excitation band around 280-300 nm and the photoluminescence band around 400-500 nm are attributed to the excitation 3d10 3d94sl and its reverse radiative deactivation 3d94s1 3 d 9 , respectively, with very small charge-transfer character. The absorption band around 300-320 nm and the photoluminescence band around 500-600 nm are attributed to the presence of a (Cu+-Cu+) dimer, Le., to the 3du* 4su excitation and its reverse radiative deactivation 4su 3du*), respectively. As shown in Figure 4 (dashed lines), the orinary photoluminescence spectrum consists of three different emissions, Le., an emission around 420 nm, a second around 470 nm, and a third around 5 15 nm. Figure 5 shows the photoluminescence spectrum at 77 K of a Cu+/ZSM-5 catalyst prepared by evacuation at 973 K. The spectrum is also well deconvoluted into three different emitting moieties, with their excitations centered around 290, 300, and 3 10 nm, respectively. In Figure 6,the synchronous scan photoluminescence (SSP) spectrum of the C u + / Z S M - 5 catalyst at 298 K is given for AT = 160 nm. The SSP spectrum also shows well-defined peaks at emission wavelengths of about 420 and 490 nm with a shoulder at 520 nm with a good coincidence in their wavelengths with those of the conventional photoluminescence spectrum obtained at 77 K. This ordinary photoluminescence spectrum at 77 K and synchronous scan photoluminescence spectrum at 298 K of the Cu+/ZSM-5 catalyst clearly indicate that there are three different types of the Cu+ emitting species on the catalyst; though, as shown in Figure 7, it is difficult to deconvolute the decay curve into three components because of their very close lifetimes around 85 ps. It is also found that the photoluminescence of these catalysts strongly depends on the evacuation temperature in its intensity and shape. As shown in Figure 2b, the intensity of the photoluminescence spectrum of a

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catalyst increases upon increasing the degassing temperature of the original Cu2+/ZSM-5 sample, passing through a maximum at 1173 K and then decreasing in the region of much higher evacuation temperatures. In the highest temperature region (where the intensity of photoluminescence became weaker), the color of the catalysts changed from white to light red, suggesting that the evacuation of the samples at temperatures higher than 1173 K leads to further reduction (Cu+ to Cuo). Thus, it is found that the relative contribution of the three emission bands to the total photoluminescence spectrum changes withdegassing temperature. At the degassing temperatures lower than 873 K, the contribution of the third emission band (5 15 nm) to the total photoluminescence was large; at degassing temperatures between 873 and 1123 K, the contribution of the second emission (470 nm) became dominant, and at degassing temperatures higher than 1 123 K, thecontribution of the first emission (420 nm) was predominant. Thus, the ease of reducibility of these three different Cu2+ species, whose presence was indicated by the EPR measurements, is quite different. Figure 8 shows a typical photoluminescence spectrum at 77 K (a) of Cu+/ZSM-5 prepared by theevacuationat 1173 K. Figure 8 also shows a typical photoluminescence spectra of the Cu+/ Y-zeolite (Figure 8b) and Cu+/SiOz (Figure 8c) for reference. These samples exhibited photoluminescence of two or three different emission bands. Cu+/SiOz shows a major band at 51 5 nm, whereas a band around 415 nm was the main component with Cu+/ZSM-5. The Cu+/Y zeolite exhibits two different intense bands around 420 and 515 nm, respectively. FT-IR and TPD Studies of the Preparation Steps of the Cu+/ ZSM-5 Catalyst and Its Interaction with NO. Figure 9 shows the IR absorption bands of the Cu2+/ZSM-5 samples which were evacuated at various temperatures. The samples degassed at temperatures below 673 K (curves 1-3) exhibit an unresolved broad IR absorption band around 3700-3000 cm-l, which can be attributed to the presence of physisorbed and/or coordinated water and to the presence of surface OH groups interacting by hydrogen bonding among themselves and/or with H2O. With increasing degassing temperature to 773 K, the sample exhibits a new band at 3604 cm-1, which is assigned to acidic OH groups.

Characterization of the Cu+/ZSM-5 Catalyst

The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5747

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Figure 8. Typical photoluminescence spectrum of the Cu+/ZSM-5 catalyst prepared by the evacuation of the Cu2+/ZSM-5sample at 1173 K (a), the Cu+/Y-zeolite catalyst prepared by the evacuation of the Cu2+/Y-zeoliteat 973 K (b), and the Cu+/SiO2catalyst prepared by the evacuation of the Cu2+/Si02 sample at 973 K (c), respectively. (The spectra were recorded at 77 K. The excitation band was around the 300-nmbeam.)

Temperature / K

Figure 10. TPD profiles of CuZ+/ZSM-5sample.

Wavenumber 1 cm-1

Figure 9. IR spectra of the Cu2+/ZSM-5samples evacuated at various degassing temperatures (in K): 1,298; 2,473; 3, 573; 4,673; 5,773; 6, 973.

0 2

(a) and H2O desorption (b) from the

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Small bands at 3656 and 3732 cm-I from isolated AI-OH and Si-OH sites, respectively, are also present. As shown in Figure 9, the intensity of the IR band due to the acidic OH groups becomes weaker upon increasing the evacuation temperature of the Cu2+/ZSM-5 samples and disappears completely upon evacuation at 973 K. Figure 10 shows a temperature-programmed desorption (TPD) profile for the desorption of 0 2 (a) and H20 (b) from a Cu2+/ ZSM-5 sample that had been dried at 273 K under vacuum. Large amounts of H20 desorb from the sample in the temperature range from 373 to 673 K; most of the H20 desorbed at 473-573 K originates from the elimination of undissociated coordinative water. As for the desorption of 02, the amounts desorbed are much smaller than those of H2O; a peak in the desorption profile of 0 2 is observed in the temperature range 463-573 K, but desorption of 0 2 is constantly observed upon degassing in the whole degassing temperature range from 673 to 1073 K. A drastic decrease in the intensity of the IR band around 37003000 cm-1 (due to coordinated H20 and/or surface O H groups interacting with adsorbed H2O) corresponds well with a drastic decrease in the intensity of the EPR signal from hydrated Cu2+ ion. From the IR spectral pattern shown in Figure 9, a small peak observed in the desorption profile of H20 observed in the temperature regions from 873 to 1073 K seems to be ascribed to the elimination of isolated surface O H groups or "free OH". The IR spectra of the Cu+/ZSM-5 catalyst in the presence of N O molecules is shown in Figure 1 1. At low N O pressure, the main band is observed at 1813 cm-l with other smaller bands a t

Figure 11. IR spectra of NO species adsorbed onto the Cu+/ZSM-5 catalyst prepared by the evacuationof the Cu2+/ZSM-5sample at 1173 K. (Theadsorption of NO was carried out at 290 K. NO pressure: 1, 0.1; 2, 2.0; 3, 3.0; 4, 5.0; 5 , 10; 6, 20 Torr. The spectra were recorded at 298 K.)

1911 and 1733 cm-1. From our previous FT-IR studies,32 the main band at 1813 cm-I can be assigned to the (Cu-NO)+ mononitosylic adduct species, characterized by a slight positive charge on the N O molecule and also by being EPR visible,32 whereas the minor bands at 191 1 cm-I can be identified as N O species adsorbed on the unreduced remaining Cu2+ions.39-42 As reported in the previous paper,32 with increasing pressures of NO, the intensity of the band at 191 1 cm-I increases and new bands attributed to a dimeric N O species with a cis configuration43 become observable at 1733 and 1827 cm-I. These results clearly indicate that the major adsorbed N O species which form in the early stages of N O uptakeonto Cu+/ZSM-5 catalyst arenitrosylic adducts (Cu-NO)+ with a weak interaction between N O and the Cu+ ion. A detailed study of the adsorption of N O molecules on the Cu2+/ZSM-5 and Cu+/ZSM-5 catalysts by ESR and FT-IR spectroscopy has previously been rep0rted.3~ Decomposition of NO into N2 and 0 2 on the Cu+/ZSM-5 Catalyst under UV Irradiation. As shown in Figure 12, UV irradiation of the Cu+/ZSM-5 catalyst even at 275 K in the presence of a pressure of N O less than 2 Torr leads to the formation of N2 and 02, with a good linear relationship between the UV irradiation time and the N O conversion. The formation of N20 and NO2 as byproducts was negligible in the photocatalytic

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Reaction Time i h

Figure 12. Reaction profiles of the photocatalyticdecomposition of NO into N2 and 02 at 275 K on the Cu+/ZSM-5 catalyst (a), Cu+/Y-zeolite catalyst (b), and Cu+/SiO2 catalyst (c), respectively. (No reaction was observed at 275 K without UV irradiation.)

decomposition reaction on the Cu+/ZSM-5 catalyst at 275 K. Thus, at lower NO pressures the photocatalytic decomposition of NO into N2and 0 2 proceeds. The yield of the photocatalytic decomposition of NO strongly increased with NO pressure. However, at higher pressures of NO (i.e., above 20 Torr), N20 was produced in addition to N2 and 0 2 . As shown in Figure 2c, the photocatalytic activity of the Cu+/ ZSM-5 catalyst for the direct decomposition of NO increases with the evacuation temperature of the Cu2+/ZSM-5 sample, passing through a maximum at 1173 K and then decreasing at much higher degassing temperatures. As shown in Figure 2b, the yields of the photoluminescence due to Cu+ are also drastically changed in the same way. It is well-known that CO molecules adsorb selectively on the Cu+ sites.44 This fact indicates that the number of Cu+ ions on the catalysts is able to be determined exactly by measuring the number of CO molecules adsorbed on the catalyst. The measurements of the selective CO adsorption on the catalysts which were prepared by degassing at higher temperatures were carried out at 295 K. In fact, the number of thereduced Cu+ionsthat can becounted by theselective titration of CO onto Cu+ sites (Figure 2d) also changes with the evaucation temperature, being parallel to the yield of photoluminescence. Figure 12also shows a reaction time profile of the photocatalytic decomposition of NO at 275 K on the Cu+/SiO2 (c) and Cu+/ Y-zeolite (b). Although it is difficult to determine the actual quantum yields in such very small powdered systems, Figure 12 shows that the Cu+/ZSM-5 catalyst exhibits the highest apparent photocatalytic activity for the direct decomposition of NO into N2 and 0 2 at 275 K. As mentioned above, with Cu+/ZSM-5 catalyst prepared by the evacuation at 1073 K, isolated Cu+ ion is the predominant species. With the Cu+/SiO2, the Cu+-Cu+ species is predominant. Therefore, the photocatalytic reactivity of the isolated Cu+ species seems to be much higher than that of the Cu+/Si02 for the direct decomposition of NO into N2 and 02.

Discussion Detection of the Three Different Types of Cuz+Ions by EPR and Their Reducibility to Cu+ Ions. Cu2+ cations anchored on ZSM-5 by ion-exchange are thought to be located on the inner channels of the zeolite as isolated Cu2+ions to which gas-phase molecules (such as H20 and NO) can be accessible.32-41 The ESR measurements clearly indicate that after the dehydration of the Cu2+/ZSM-5 samples at temperatures above 373 K, the samples exhibit well-resolved EPR signals which consist of three different Cu2+ species (species A-C) with slightly different gand A values. Small changes in the donor-acceptor properties of the ligand environment of Cuz+ and/or geometrical displacement of the Cu2+site result in remarkable changes in the intensity and hyperfine structure of the EPR signal assigned to Cu2+. Increasing the evacuation temperature of the Cu2+/ZSM-5 samples causes the EPR signals of Cuz+ to change to well-resolved, structured signals, suggesting that the ligand sphere of the Cu2+ site

Anpo et al. progressively changes from H20 to O H groups and then partly to lattice 0 2 - ions of the zeolite. Recent XANES and EXAFS analysis of copper ion-exchanged ZSM-5 has also revealed that copper atoms in ZSM-5 are more ionic than CuO and are almost atomically dispersed in the zeolite cages.45 Further evacuation at higher temperatures results in the stabilization of the Cu2+ions mainly a t two different sites of the zeolite by further substitution of OH ligands by lattice 02-.In agreement with these results, Slinkin et a1.46.47have measured EPR signals for Cu2+/ZSM-5samples calcined at 1073 K, finding two discrete but similar types of isolated Cu2+ ions (gll = 2.32, All = 159, g, = 2.05, gl = 2.31, ,411= 159 G,gl = 2.05). These two Cuz+ions are known to be anchored onto two different cationexchangeable sites in ZSM-5: (1) sites close to the wall in the main channel and (2) sites recessed from the main channel in the side pockets (inside five-membered rings). Although there are some inconsistencies in EPR parameters assigned to the Cu2+ion in past literature, similar EPR parameters have also been reported with Cu2+/ZSM-5 by several groups.33,3494* Thus, two of the three different types of the Cu2+species (species A and B) can be assigned to Cuz+ ions located at sites close to the wall in main channels and at sites recessed from the main channels in side pockets, respectively. Regarding the other Cu2+ species (species C), as mentioned above, this species is more easily reduced at lower temperatures than species A and B, and its interaction with the ZSM-5 network seems to be rather weak. This suggests that the third Cu2+center corresponding to species C may be coordinated on the outer surface of ZSM-5 and have primary interaction with the outer component of coordinated H2O and/or surface OH groups. Detection of the Three Different Types of Cu+ Species by Photoluminescence Measurements. The ordinary photoluminescence spectra and the corresponding excitation spectra of the Cu+/ZSM-5 catalysts (Figure 4 and 5) clearly indicate that there are three Cu+ emitting sites (emitting sites X, Y, and Z) on the catalysts. The synchronous photoluminescencespectrumobtained at 298 K (Figure 6) also clearly shows that there are three Cu+ emitting species on the catalysts. Given the previous results obtained with the C u + / Y - z e ~ l i t e , ~Cu+/ZSM-5,49-S0 ~~~"~~ Cu+/ A1z03,24-27,51 and Cu+/SiOz,zOJ the photoluminescence band around 520 nm (with an excitation band around 3 10 nm (emission Z)) may be attributed to the radiative decay from the electronic excited state of the Cu+-Cu+ dimer species (4sa)*. On the other hand, two other photoluminescence bands around 420 (emission X) and 470 nm (emission Y) are attributed to the radiative decay from the electronic excited state of the isolated Cu+ species (3d94s1)* having slightly different electronic states due to their different environments. As mentioned above, the fractional contribution of these three emissions (X, Y, and Z) to the total photoluminescence spectra depends on the evacuation temperature used for the preparation of the catalyst. Emission Z can be observed first with Cu+/ ZSM-5 which had been evacuated at the lowest temperature regions, suggesting the reducibility of the Cuz+ ions to Cu+. Therefore, together with the results obtained by EPR measurement of the Cu2+species C, the emitting species is easily linked to the formation of Cu+ ions by reduction of the Cuz+ions located on the outer surfaces (having surface OH groups and/or coordinated H20 as ligands). During evacuation, these Cuz+ ions are easily reduced to Cu+ and are easily mobilized on the surfaces to form the Cu+-Cu+ dimer. The contribution of the emission moieties X and Y to the total photoluminescence becomes significantly larger with the Cu+/ ZSM-5 samples which had been degassed at temperatures above 873 K when the decrease in EPR intensity of the Cuz+ species A and B becomes significant. These results suggest that the emitting species X and Y may correspond to the presence of the Cu2+ ions of type A and B, respectively, located in sites close to the main channels and also a t sites recessed from the main channels in side pockets, respectively. The reduction of these Cu2+ ions

The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5749

Characterization of the Cu+/ZSM-5 Catalyst ‘

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Temperature / K

Figure 13. Effects of the H2 reduction temperature of the Cu*+/ZSM-S sample on the relative intensitiesof the EPR signal due to Cu2+ions (a), of the photoluminescence due to Cu+ ions (b), and the amount of CO molecules selectively adsorbed on Cu+ ions of the catalyst at 285 K (c). (The spectra were recorded at 77 K.)

to Cu+ by thermoevacuation requires relatively high temperatures: therefore, the emission components X and Y from isolated Cu+ ions areobservableonlyafter evacuationof theCu2+/ZSM-5 sample at higher temperatures. Figure 13 shows the effect of the hydrogen reduction temperature of the Cu2+/ZSM-5 samples on the intensity of the EPR signal assigned to theCu2+ions and of the photoluminescence spectra attributed to the presence of Cu+ ions, respectively. Increasing reduction temperatures of the Cu2+/ZSM-5 samples led to a decrease of the EPR signal intensity due to Cu2+ and to a simultaneous increased intensity of the photoluminescence spectra due to Cu+. The EPR signal from species C disappeared first a t relatively low reduction temperatures, as did the photoluminescence spectrum of the emitting species Z. Thus, the results obtained with the hydrogen reduction treatment clearly indicate that the photoluminescence component Z is closely associated with the presence of Cu2+ species C, in good agreement with results obtained by the thermal evacuation. These results also indicate that there is a gap between the temperature where the decrease in the EPR signal starts and that where the increase in the intensity of the photoluminescence begins (Figure 2). This effect can be explained by the following possibilities: (1) dehydration of the Cuz+ ions (changing the ligands from H 2 0 to OH groups or even to lattice oxygen, 02-), which leads to a drastic decrease in the intensity of the ESR signals, as mentioned above; (2) efficient quenching of photoluminescence of the formed Cu+ by surface OH groups and/or 0 2 adsorbed on zeolite which exist near Cu+ species and desorbed only by evacuation treatment higher than 673 K. Role of Cu+ Ions in the Decomposition of NO into Nz and 02 at 275 K on the Cu+/ZSM-5 Catalyst under UV Irradiation. Figure 2 shows that the yield of the photocatalytic decomposition of N O parallels the yield of the photoluminescence of Cu+ and the number of Cu+ species determined by the selective adsorption of CO. These results clearly indicate that the presence of Cu+ plays a significant role in the photocatalytic decomposition reaction of N O on the Cu+/ZSM-5 catalyst. Figure 13 also shows a reaction time profile of the photocatalytic decomposition of N O at 275 K on the Cu+/SiO2 and Cu+/Yzeolite. Although it is difficult to determine the actual quantum yields in such very small powdered systems, the Cu+/ZSM-5 catalyst exhibits the highest apparent photocatalytic activity for the direct decomposition of N O into N 2 and 0 2 at 275 K. As mentioned above, in the Cu+/ZSM-5 catalyst prepared by the evacuation at 1073 K, isolated Cu+ ions are the predominant species, whereas with the Cu+/Si02,the Cu+-Cu+ species, species Z, is predominant. Therefore, the photocatalytic reactivity of Cu+/ZSM-5 (mainly containing isolated Cu+ species) is much higher than that of the Cu+/Si02 for the direct photocatalytic decomposition of N O into N2 and 0 2 . Figure 14 shows the effect of the addition of N O at 293 K on the photoluminescence spectrum of the Cu+/ZSM-5 catalyst that

Figure 14. Effect of the addition of NO on the photoluminescence spectrum of the Cu+/ZSM-5 catalyst which was prepared by the evacuationof the Cu2+/ZSM-5sample at 1173 K. (The addition of NO was carried out at 285 K. NO pressure (in Torr): 1,O.l; 2, 0.3; 3,O.S; 4, 1; 5,20.) (The excitation spectra were monitored at 450 nm emission (b) and 520 nm emission (c), respectively.)

had been prepared by the evacuation of the Cu2+/ZSM-5sample at 1173 K. The addition of N O onto the Cu+/ZSM-5 catalyst leads to the efficient quenching of the photoluminescence due to Cu+. The lifetime of the photoluminescence was shortened by the addition of NO, its value becoming shorter with increasing pressures of NO, Le., from about 85 p s in vacuum to 70 p s in the presence of 1.O Torr pressure of NO molecules. The evacuation of the system after the quenching of the photoluminescence led to the complete recovery of the photoluminescence to its original intensity and lifetime levels. These results clearly suggest not only that the interaction of N O with the Cu+/ZSM-5 catalyst is weak but also that the added N O molecules easily interact with the Cu+ species both in its ground and excited states. Both the EPR signal and the FT-IR spectrum observed in the presence of N O indicate that in the presence of low pressure of N O molecules a nitrosylic adduct, i.e., (Cu-NO)+, is the major adsorbed species. As described in our previous paper,3l the stretching frequency of the (Cu-NO)+ species (at 18 12 cm-I) indicates that a partial electron transfer from the Cu+ ion to N O molecule occurs, resulting in a slightly negatively charged nitrosylic adduct species, Cu+*+-NO&. UV irradiation of the Cu+/ZSM-5 catalyst with (Cu-NO)+ species on it led to a decrease in the intensity of the EPR signal and IR spectrum assigned to (Cu-NO)+ species with UV irradiation time without the appearance of any new EPR signals and IR peaks. After the UV irradiation was stopped, the intensities of the EPR signal and of the IR spectrum returned to their original levels. These reversible changes in the EPR signal and IR spectrum assigned to (Cu-NO)+ suggest not only that (Cu-NO)+ species act as reaction precursors but also that the photoinduced decomposition of NO molecules proceeds catalytically. From these results, the mechanism of the decomposition of N O into N2 and 0 2 on the Cu+/ZSM-5 catalyst at 275 K under UV irradiation is proposed as follows (Scheme 1): electron transfer from the excited state of the Cu+ ion (3d94sl state) to a a-antibonding orbital of NO and simultaneous electron transfer from the a-bonding orbital of NO to the vacant electron state of the Cu+ ion (3d94s0state) occur, causing local charge separation and a weakeningof theN-0 bond that initiates thedecomposition of NO. Twodifferent N O adsorbates, Le., N O which constitutes the (Cu-NO)+ adduct and N O which is supplied from the gas phase, are simultaneously activated at a Cu+ site in the local electron transfer, resulting in the selective formation of N2 and O2 without any formation of N 2 0 and/or NO2 in the photocatalytic decomposition of N O on the Cu+/ZSM-5 catalyst at 275 K. However, the mechanism involving two relatively close Cu+ sites also seems possible. Conclusions The in situ ordinary and synchronous scan photoluminescence, EPR, and IR measurements on the Cu+/ZSM-5 catalyst in the

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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994

Anpo et al.

SCHEME 1: Reaction Scheme of the Photocatalytic Decomposition of NO into N2 and 0 2 on the Cu+/ZSM-5 Catalyst at 275 K

cu2+ 0 ' '0

Cu'

evac

& reduction

I

-

Iwamoto, M.; Hamada, H. Catal. Today 1991, IO, 5 7 . Armor, J. N. Appl. Catal. 1992, I , 221. Iwamoto, M.; Mizuno, N. J. Auto, Eng. 1993, 207, 23. Teraoka, Y.; Kagawa, S. Hyomen 1993, 31, 913. Chin, A. A.; Bell, A. T. J . Phys. Chem. 1983, 87, 3700. Held, W.; Koenig, A.; Richte, T.; Puppe, L. SAE Paper 1990,900496. Misono, M.; Kondo, K. Chem Lett. 1991, 1001. Kikuchi, E.; Yogo, K.; Tanaka, S.; Abe, M.Chem. Lett. 1991,1063. Inui, T.; Kojo, S.; Yoshida, T.; Iwamoto, S. Caral. Lett. 1992, 16,

(3d")

\

0

0

I

1

m

?J

Monomer

(N--oIad

:&-Jtlon

(11) Kagawa, S.; Ogawa, H.; Furukawa, H.; Teraoka, Y. Chem. Lett. 1991, 407.

NO s-

,,

c'uAc-

2"

-1-ILl

electron transfer into n-anti-bonding orbital of NO

nitrosyl

Cu+*(3d94s')

excited state

presence and absence of N O allowed the description of the processes of Cu+ formation from Cu2+ and also evidenced the presence of three different anchored Cu+ species as well as their location in the ZSM-5 network. The Cu2+species located in the sites close to the main channels and those in sites recessed from the main channels in side pockets were found to be rather stable and could be reduced to Cu+ only by the evacuation at temperatures above 773 K. The Cu+ species located in the main channels and in the side pockets exhibited characteristic photoluminescence around 400-450 and 420-480 nm, respectively. The evacuation of the Cu2+/ZSM-5 system at 1073 K led selectively to a Cut/ZSM-5 catalyst with the highest yield of Cu+. This catalyst exhibited the highest photocatalytic reactivity for the direct decomposition of NO into N2 and 0 2 at 275 K. A good parallel between the yields of the photoluminescence (due to the presence of Cu+) and the yields of the photocatalytic decomposition of N O was found, indicating that the excited state of Cu+ plays a significant role in the decomposition of N O into N2 and 0 2 under UV irradiation. The direct observation of the (Cu-NO)+ species and its behavior under UV irradiation (carried out by EPR and IR spectroscopy) indicates that local electron transfer from an excited Cut ion to the a-antibonding orbital of N O and simultaneous electron transfer from the a-bonding orbital of N O to the vacant orbital of the Cu+ ion (3d9) lead to the direct decomposition of two N O molecules on a Cu+ site to selectively form N2 and 0 2 under UV irradiation, even a t 275 K. These results suggest that the direct decomposition of N O into N2 and 02on the Cu+/ZSM-5 catalyst under UV irradiation is a new type of photocatalysis achieved within the small cavities of the zeolite. Acknowledgment. This work has been supported by the Ministry of Education of Japan for Grant-in-Aid Scientific Research (Grant No. 03650659) and International Joint Project Research (Grant No. 02044125). M.A. is much indebted to Osaka Prefecture for Special Project Research and to Takuma Research and Development Co., Ltd., for financial support. M.A. is also much indebted to Tosoh Corp. for supplying ZSM-5 samples. This work on photocatalysis at the University of Texas is supported by the U S . Department of Energy, Office of Basic Energy Science. M.A. thanks the Universite P. et M. Curie for a position as invited Professor at Paris. Thanks (M.A. and H.Y.) are due to the Japan/U.S. Cooperative Program in Photoconversion and Photosynthesis Research. References and Notes (1) Hightower, J. W.; Van Leirsburg, D. A. In The Catalytic Chemistry ofNitrogen Oxides; Klimish, R. L., Larson, J. G., Ed.; Plenum: London, 1975; p 63.

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