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Characterization and NO decomposition activity of Cu-MFI zeolite in relation to redox behavior
Applied Catalysis A: General 200 (2000) 167–176
Characterization and NO decomposition activity of Cu-MFI zeolite in relation to redox behavior Y. Teraoka∗ , C. Tai, H. Ogawa1 , H. Furukawa, S. Kagawa Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852-8521, Japan Received 24 February 2000; received in revised form 10 April 2000; accepted 11 April 2000
Abstract The redox behavior and states of Cu ions in Cu ion-exchanged MFI (Cu(n)-MFI, n: exchange level) have been investigated by means of temperature-programmed desorption (TPD) of oxygen, diffuse reflectance (DR) UV–VIS spectroscopy and Cu+ photoluminescence (PL) spectroscopy. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance of three desorption peaks: ␣ (below 200◦ C),  (300–500◦ C) and ␥ (above 500◦ C). It has been suggested that ␣ and  oxygen are extra-lattice oxygen adsorbed on Cu ions, while ␥ oxygen is lattice oxygen coordinated to Cu ions. The Cu+ emission was tremendously reduced once the catalyst contacted with O2 and NO at elevated temperatures such as 500◦ C, and it was almost invisible under the working state of the catalyst, suggesting that PL-active Cu+ ions are not real active sites under the working state. The desorption of  oxygen was intimately related to the creation of active centers for the NO decomposition reaction. DR measurements showed that the desorption of  oxygen caused tetragonal Cu2+ to decrease and trigonal Cu2+ to increase simultaneously. It has been proposed that both Cu2+ and Cu+ are involved in the NO decomposition catalysis over Cu-MFI under the working state. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Cu-MFI zeolite; Nitric oxide decomposition; Redox behavior; Active site
1. Introduction Iwamoto et al. first reported the high and stationary activity of Cu ion-exchanged MFI zeolite (Cu-MFI) for the direct decomposition of NO into N2 and O2 [1–3]. These pioneering works, coupled with the discovery of the selective reduction of NOx with hydrocarbons [4,5], have triggered a huge number of related studies, and have contributed to the establishment of a field of NOx catalysis with Cu-loaded zeolites. Apart ∗ Corresponding author. Tel.: +81-95-848-9652; fax: +81-95-848-9652. E-mail address: [email protected] (Y. Teraoka). 1 Present address: Nanyo Research Laboratory, Tosoh Corporation, Shinnanyo 746-8501, Japan.
from original papers, many review articles on this subject have been published [6–12]. There are many common features of the catalytic behaviors of Cu-MFI for the direct NO decomposition. The reaction is a true decomposition [13,14]; it passes through a maximum around 500◦ C; and it is inhibited by oxygen. Copper is the most active or practically the only ingredient in zeolitic NO decomposition catalysts, and the specific activity of exchanged Cu ion increases with decreasing the Al content of host zeolites [2]. The activity increases with an increase in the Cu content, and over-exchanged catalysts are more active than under-exchanged catalysts [3,15]. Reaction mechanism, intermediates and active Cu site/species concerning the NO decomposition over Cu-MFI have been extensively investigated
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by such techniques as IR, EPR, XPS, XAFS, Cu+ phosphorescence, diffuse reflectance UV–VIS and temperature-programmed methods. The details will be left to review articles [6–12], but the real active/working states of Cu-MFI catalysts are still controvertible: for example, some contradictory reaction mechanisms were reported, such as redox mechanism with binuclear active center (a pair of Cu+ ins or [Cu–O–Cu]) [2,14,16] and non-redox mechanism with isolated Cu2+ ion [17]. We have previously proposed that both Cu2+ and Cu+ are involved in the NO decomposition catalysis over Cu-MFI catalysts [18]. In this paper, the redox behavior of Cu ions in the MFI zeolite under various treatments is investigated by means of temperature-programmed desorption (TPD) of oxygen, photoluminescence and diffuse reflectance UV–VIS spectroscopy. The relation between the redox behavior and the NO decomposition catalysis is also discussed.
2. Experimental 2.1. Catalysts Na-MFI (SiO2 /Al2 O3 =23.3, Tosho Corporation) was treated with 0.1 M aqueous NaNO3 overnight at 60◦ C, followed by filtration and a conventional ion-exchange procedure using aqueous Cu(II) acetate at the same temperature [3]. After the ion exchanges, samples were filtered, washed thoroughly with deionized water, and dried in an oven at 110◦ C. The samples thus obtained were referred to as Cu(n)-MFI; n in parentheses means the content of Cu in terms of ion exchange level, which was determined by atomic absorption spectroscopy after dissolving in an aqueous HF solution: the stoichiometry of Cu/Al=0.5 corresponds to 100% exchange level. Samples were treated in various ways before subjecting them to characterization experiments. The as-synthesized (fresh) samples were evacuated, exposed to O2 (100 Torr) and re-evacuated at 500◦ C (0.5 h for each treatment); this is designated the standard pre-treatment. Two types of oxygen adsorption treatment were employed; after exposure to O2 (usually 100 Torr) for 0.5 h at T1 ◦ C, samples were evacuated at the same temperature ([Ox, T1 ]) or cooled in the O2 atmosphere to T2 ◦ C ([Ox, T1 →T2 ]). The
samples were also subjected to the evacuation treatment at T◦ C ([Evac, T]). If necessary, the starting samples are indicated by subscript ‘p’ and ‘f’ for the samples after the standard pre-treatment and the fresh sample, respectively; for example, [Evac, 800]p and [Evac, 800]f mean the evacuation treatments at 800◦ C of pre-treated and fresh samples, respectively. 2.2. NO decomposition reaction The steady-state NO decomposition reaction was carried out in a fixed-bed flow reactor. The reaction gas (1 vol.% NO in He) was fed at a rate of 15 cm3 min−1 over 1.0 g catalyst which was pre-treated at 500◦ C for 1 h in a helium stream. The temperature dependence of the NO decomposition reaction over Cu(130)-MFI is depicted in Fig. 1 as a representative example. The pulse decomposition of NO was performed as follows. Catalysts (0.05 g) were packed in a reactor made of stainless steel tubing, and under He flowing at a rate of 20 cm3 min−1 a small portion of NO gas (3.8 mol) was pulsed. The reaction products of both steady-state and pulse reactions were analyzed by gas chromatography (TCD). 2.3. Characterization Temperature-programmed desorption (TPD) of oxygen was carried out in a stream of He at a heating rate of 10◦ C min−1 , and the desorbed oxygen was
Fig. 1. Temperature dependence of NO decomposition reaction over Cu(130)-MFI. NO(1%)–He(balance). W/F=4.0 g s cm−3 .
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monitored by a TCD detector. When an equimolar mixture of 16 O2 and 18 O2 was used as the adsorption gas, a quadrupole mass spectrometer (QMS, Nichiden Varian TE-600) was used to monitor desorbed gaseous components. Diffuse reflectance (DR) UV–VIS spectra were recorded at room temperature (RT) on a Shimadzu UV-3100 spectrometer equipped with an integrating sphere using BaSO4 as a reference material; the data were processed according to Kubelka–Munk theory. The photoluminescence (PL) spectra were measured at RT employing a Nihonbunko FP770 spectrometer.
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Fig. 3. Desorption amounts of ␣,  and ␥ oxygen per Cu ion from Cu(n)-MFI as a function of Cu ion-exchange level.
3. Results and discussion 3.1. Oxygen desorption behavior TPD chromatograms of oxygen from Cu(n)-MFI are shown in Fig. 2; oxygen adsorption was performed by cooling the Cu(n)-MFI in O2 (100 Torr) from 500◦ C to RT after the standard pre-treatment at 500◦ C, [Ox, 500→RT]p . The amount of oxygen desorbed was a trace for Na-MFI (n=0) and increased with an increase in the Cu content n, indicating that Cu ions are responsible for the emergence of the oxygen desorption. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance, in principle, of three desoprtion peaks, ␣,  and ␥. As can be recognized
Fig. 2. TPD chromatograms of oxygen from Cu(n)-MFI (n=0, 35, 48, 78 and 130). Oxygen adsorption, [Ox, 500→RT]p .
from the desorption chromatograms of Cu(78)-MFI, ␣ peak below 200◦ C seems to consist of two peaks, but it is tentatively considered to be one desorption peak in this study. With an increase in the Cu content, the amount of oxygen desorbed under  and ␥ peaks increased, and the onset and peak temperatures of  and ␥ peaks decreased and increased, respectively. For the samples with larger n (78, 130%), ␣ peak was additionally observed below 200◦ C. The oxygen desorption behavior observed in this study is essentially consistent with that so far reported [19–21]. In Fig. 3, the amounts of each desorbed species per Cu ion (specific amount) are plotted as a function of the ion-exchange level. The total amount of oxygen desorbed up to 750◦ C corresponded to 0.08 (n=35), 0.09 (48), 0.13 (78) and 0.15 (130) O2 /Cu, indicating that oxygen desorption/adsorption capacity per Cu ion increases with increasing the exchange level of Cu. The specific amount of ␥ oxygen was almost constant irrespective of the ion-exchange level. Judging from the fact that the amount of ␥ oxygen depended almost exclusively on the amount of Cu ions loaded, one may conclude that ␥ oxygen is lattice oxygen coordinated to Cu ions, as also suggested by Valyon and Hall [19]. The specific amounts of ␣ and  oxygen, on the other hand, depended significantly on the ion-exchange level of Cu ions, and they must be the adsorbed species on Cu ions or the extra-lattice oxygen (ELO) [19–21]. In the following, the nature of ␣ and  oxygen was investigated in more detail. Fig. 4 depicts TPD chromatograms of oxygen from Cu(130)-MFI below 550◦ C after the treatments of [Ox, 500→RT]p (1), [Ox, 100→RT]p (2) and [Ox, 300]p
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Fig. 4. TPD chromatograms of oxygen from Cu(130)-MFI. Oxygen adsorption: (1) [Ox, 500→RT]p , (2) [Ox, 100→RT]p , (3) [Ox, 300]p .
(3). As described above, the desorption of ␣ and  oxygen was separated below 200◦ C and above 300◦ C, respectively, with the [Ox, 500→RT]p treatment (1). It can be seen that selective formation of ␣ and  oxygen was realized by the oxygen adsorption treatments of [Ox, 100→RT]p (2) and [Ox, 300]p (3), respectively. The amount of ␣ oxygen after the [Ox, 100→RT]p treatment (in the absence of  oxygen) was far larger than that after the [Ox, 500→RT]p treatment (in the presence of  oxygen). This strongly suggests that Cu ions in the MFI zeolite can adsorb both ␣ and  oxygen depending on the oxygen adsorption treatment; in other words, there are no specific Cu sites adsorbing ␣ or  oxygen. TPD experiments were also carried out using an equimolar mixture of 16 O2 and 18 O2 after the selective formation of ␣ ([Ox, 100→RT]p ) and  ([Ox, 300]p ) oxygen. In both cases, the main desorbed species was 16 O18 O, and the compositions of desorbed O2 , [16 O18 O]2 /[16 O2 ][18 O2 ], were close to 4 in the temperature ranges of both ␣ and  desorption, indicating that the homomolecular exchange of gaseous molecules reached the equilibrium. The 18 O fraction in the temperature range of the ␣ desorption was almost constant (0.50–0.48) and close to that of the charged gaseous mixture (0.50), while that of the  desorption was smaller than 0.50 and decreased with increasing temperature (0.40 at 300–350◦ C, 0.25 at 400◦ C, 0.14 at 450◦ C). From these results, it can be concluded
that  oxygen exchangeable with lattice oxygen is a dissociatively adsorbed species, and this must be the atomic ELO bridging two Cu ions [16,19]. The nature or real form of ␣ oxygen, which is not exchangeable with lattice oxygen but can contribute to the equilibration of the homomolecular exchange reaction, is not clear at present. Valyon and Hall assumed that the low-temperature species (␣ oxygen) was molecularly adsorbed oxygen [19]. It might be also possible to assume that ␣ oxygen is atomic species because the homomolecular exchange reaction generally proceeds with the participation of atomic adsorbed species; the temperature is too low for ␣ oxygen to exchange with the lattice oxygen. As stated above, the ␣ peak is composed of two peaks, and therefore one might correspond to the molecular species and the other to the atomic species. Since ␣ oxygen is not so important for the creation of the real active centers for NO decomposition reaction (vide infra), no further investigation was carried out on the nature of ␣ oxygen. The adsorption site of ␣ and  oxygen is the same as stated above, but the adsorption state and binding energy are different from each other because of the different desorption temperatures. The  oxygen bridges two Cu ions, while we consider, at present, that an ␣ oxygen is adsorbed on one Cu ion. The formation of the bridging ELO ( oxygen) would require the migration and access of Cu ions. When oxygen is adsorbed at 300◦ C or by cooling from 500◦ C to RT,  oxygen is preferentially formed because the temperature is enough high for Cu ions to migrate, come close to each other and to be bridged by the atomic ELO. In oxygen adsorption at lower temperatures such as [Ox, 100→RT], Cu ions can no longer migrate, so that oxygen is adsorbed on one (isolated) Cu ion. If ␣ oxygen is a molecularly adsorbed species, one additional possibility is that the temperature is too low for the oxygen molecule to be dissociated. When the Cu ion-exchange level became high enough, isolated Cu ions might be left after adsorbing  oxygen and they might be the adsorption sites of ␣ oxygen. This would be the case for Cu(78)- and Cu(130)-MFI in Fig. 2. 3.2. Relationship between oxygen desorption behavior and NO decomposition activity Comparison between Figs. 1 and 2 shows that the take-off temperature of the NO decomposition
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Fig. 5. Relation between the amount of  oxygen desorbed and the NO decomposition activity at 450◦ C. Catalysts, Cu(n)-MFI. Numerals in the figure are ion-exchange level n in Cu(n)-MFI.
into N2 and O2 is well coincident with that of the  desorption. It was reported that the NO decomposition activity of Cu-MFI showed the ‘S-shaped’ dependence on the exchange level of Cu ions [15]. When the NO decomposition activity was plotted against the amount of  oxygen (Fig. 5), on the other hand, an almost linear relation was observed between the two quantities in the range covering under- and over-exchanged levels. These results strongly imply, though indirectly, that Cu ions/sites carrying  oxygen should be the active center for the direct NO decomposition when they release  oxygen. In order to confirm this, the pulse NO decomposition was carried out using Cu(76)-MFI catalyst. Fig. 6(A) and (B) show the results at 300◦ C at which both N2 and N2 O are formed in the steady-state reaction (Fig. 1)
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and the desorption of  oxygen hardly takes place (Fig. 2). The catalyst in Fig. 6 (A) was treated by [Ox, 500→300]p and therefore carried  oxygen, while the catalyst free from  oxygen, Fig. 6(B), was obtained by heating in a helium stream at 500◦ C for 0.5 h before subjecting the pulse reaction. Over the catalyst with  oxygen (Fig. 6(A)), the formation of N2 was never detected, and that of N2 O reached the steady value after the fourth pulse. In case of the catalyst without  oxygen (Fig. 6(B)), on the other hand, the formation of N2 was observed at the first three pulses; the amount of N2 formed progressively decreased with increasing the pulse number and none was detected after the fourth pulse. With increasing the pulse number, the N2 O formation reached a maximum at the second pulse, decreased progressively (second to sixth pulses) and eventually reached the steady value equal to that in Fig. 6(A) after the sixth pulse. After the tenth pulse of NO, the catalyst was cooled down to RT and the desorption of gaseous molecules was monitored by QMS while heating in the He stream at 10◦ C min−1 . Oxygen was a sole desorbed species in the range between RT and 550◦ C, and the desorption temperature (>300◦ C) completely agreed with that of  oxygen. The sum of NO converted into N2 in Fig. 6(B), 2.4 mol, was reasonably close to the amount of  oxygen, 2.6 mol, calculated for 0.05 g of Cu(76)-MFI charged in the pulse reactor. These results indicate that oxygen atoms resulting from the direct NO decomposition into N2 accumulate in the catalyst as  oxygen. Accordingly, the catalyst after the fourth pulse in Fig. 6(B) is expected to carry  oxygen to its capacity. This is supported by the
Fig. 6. Pulse decomposition of NO over Cu(76)-MFI at 300◦ C (A,B) and 500◦ C (C). Pre-treatment: (A) [Ox, 500→300]p , (B) He-purge at 500◦ C for 0.5 h, (C) standard pre-treatment.
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resemblance of the pulse reaction results of Fig. 6(B) after the fourth pulse and Fig. 6(A) with respect to the dependence on the pulse number and the reaction products. When the pulse reaction was carried out at 500◦ C (Fig. 6(C)), only the formation of N2 was observed at higher conversion level. This is reasonable, because at 500◦ C the conversion of NO into N2 O is never observed in the steady-state reaction (Fig. 1) and the spontaneous desorption of  oxygen is expected to proceed under the He stream between the NO pulses. These results confirm the strong implication of  oxygen or Cu ions carrying  oxygen to the direct NO decomposition catalysis. When  oxygen desorbs spontaneously or thermally, the resulting site becomes the active center of the NO decomposition reaction into N2 and O2 . Considering that  oxygen is bridging two Cu ions, at least two Cu ions are necessary to form the active center (see below for more detail). The pulse reaction results also show that the active site for the N2 O formation is different from that for the N2 formation at 300◦ C. 3.3. Spectroscopic characterization Since Iwamoto et al. pointed out the importance of Cu+ ions and the redox change between Cu+ and Cu2+ for the appearance of the stationary NO decomposition activity [2,16], photoluminescence (PL) technique has often been used to detect Cu+ in zeolite matrices [7,22–26]. Fig. 7 shows the Cu+ emission spectra of Cu(83)-MFI after various treatments. When the fresh catalyst was evacuated at various temperatures, [Evac, T]f , no Cu+ emission was observed below 300◦ C, a less resolved spectrum at 400◦ C (Fig. 7(A)) and a well-resolved spectrum with maxima at 480 and 540 nm above 500◦ C (B, C). The bands at 480 and 540 nm were assigned to Cu+ in close proximity to the two and one framework Al, respectively [23]. With an increase in the evacuation temperature, the intensity of the 540 nm band passed a maximum at 600◦ C, while that of the 480 nm band leveled off above 600◦ C. The dependence of the Cu+ emission on the ion-exchange level (n) was examined with Cu(n)-MFI (n≥35) after the [Evac, 600]f treatment. The 480 nm band was observed even with Cu(35)-MFI and the intensity increased moderately with increasing n. The 540 nm band intensity, on the other hand, was vary
weak at n<50 and increased tremendously with increasing n. The 480 and 540 nm bands predominated at ion-exchange level below and above ca. 80%, respectively. The dependencies of the Cu+ emission on the evacuation temperature and the Cu content were essentially consistent with those reported [7,23]. It should be noted here that the strong Cu+ emission was observed only when the as-prepared (fresh) Cu-MFI was evacuated at higher temperatures and that the Cu+ emission tremendously weakened once the Cu-MFI was contacted with oxidizing gases such as O2 and NO at elevated temperatures like 500◦ C. As seen from spectrum D in Fig. 7, the Cu+ emission almost disappeared after the standard pre-treatment at 500◦ C, that is, the evacuation, oxidation and re-evacuation at 500◦ C. Judging from the TPD result described in Section 3.1, one would expect that a substantial amount of Cu+ is produced when the pre-treated sample is evacuated at 800◦ C; the desorbed oxygen should amount to more than 2.5 O/Cu. As can be seen from Fig. 7E, however, the Cu+ emission intensity was very low. This indicates that under the working state the population of PL-active Cu+ ions is low and thus the photoluminescence study gives rather little information about the steady or working state of Cu-MFI, although Cu+ ions detected by PL spectroscopy are active for the NO decomposition and should be the precursor of the active center for NOx abatement catalysis (vide infra). EPR and DRS are known to be powerful tools to investigate the existing state of Cu2+ ions [27], but the
Fig. 7. Cu+ emission spectra of Cu(83)-MFI (Ex=280 nm). Pre-treatment: (A) [Evac, 400]f , (B) [Evac, 500]f , (C) [Evac, 800]f , (D) standard pre-treatment, (E) [Evac, 800]p .
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Fig. 8. Diffuse reflectance UV–VIS spectra of Cu(130)-MFI. Pre-treatment: (1) standard pre-treatment, (2) [Ox, 100→RT]p , (3) [Ox, 500→RT]p .
application of DRS to Cu-MFI has been very limited as compared with uses of EPR [28,29]. Fig. 8 depicts DR spectra of Cu(130)-MFI after various treatments. As can be recognized from the TPD results, both ␣ and  adsorbed oxygen are absent after the standard pre-treatment (1), the [Ox, 100→RT]p treatment (2) results in the selective formation of ␣ oxygen, and the [Ox, 500→RT]p treatment (3) gives both  (major) and ␣ (minor) oxygen. Five absorption bands were observed depending on the treatments: A1 at ca. 240 nm, A2 at ca. 320 nm, A3 at ca. 440 nm, A4 between 600 and 900 nm and A5 at ca. 1640 nm. The A4 band consisted of several peaks, but here they are collectively termed the A4 band (see below for the detail). Only the A1 band was observed with Na-MFI which might originate from the framework Al–O units [30], but its intensity, F(R∞ )=0.5 at the peak maximum, was very weak as compared with that of Cu-MFI. In addition, since the 3d10 →3d9 4 s1 transition band of Cu+ , which is used for the excitation of Cu+ photoluminescence, appears around 280 nm [30,31], the contribution of Cu+ , if present, to the A1 and A2 bands should be taken into account. However, this effect might be negligible in Fig. 8 because all the samples experienced the standard pre-treatment at 500◦ C, which causes the tremendous decrease in the Cu+ emission (Fig. 7) and excitation (not shown). Accordingly, five bands observed should be related to Cu2+ ions. As stated below, A1 , A2 and A3 bands are the ligand-to-metal
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charge transfer (LMCT) transition from oxygen to Cu2+ , while A4 and A5 are the d–d transition of Cu2+ . It is known that the LMCT of O→Cu2+ occurs in the range of 230–255 nm for Cu ion-exchanged zeolites, which includes all the possible LMCT transitions irrespective of the symmetry of the Cu2+ complex [27]. Since Cu(130)-MFI after the standard pre-treatment (1) is considered to carry no ELO (neither ␣ nor  oxygen), most of the A1 band should be contributed by the LMCT from lattice oxygen to Cu2+ ion. Actually, the A1 -band intensity of Cu(n)-MFI after the standard pre-treatment increased with increasing n. The A2 band appeared after the [Ox, 100→RT]p treatment (2) and the A3 band by the [Ox, 500→RT]p treatment (3), strongly suggesting that the A2 and A3 bands have some relation to ␣ and  oxygen, respectively. Since d–d transitions of Cu2+ occur above 500 nm, the A2 and A3 bands could be assignable to LMCT bands from ␣ oxygen and  oxygen to Cu2+ ion, respectively. The increase of the A1 -band intensity by the oxygen adsorption treatment as well as the order of the intensity: 1<2<3 may reflect the fact that oxygen adsorption causes an increase in the Cu2+ concentration by the oxidation of Cu+ to Cu2+ . The position of the LMCT band maximum (ν in unit of cm−1 ) can be related to the difference in optical electronegativity of the oxygen and Cu2+ , χ opt (O) and opt (Cu2+ ) [32]. ν (cm−1 ) = 30000[χopt (O) − χopt (Cu+2 )]
(1)
Since the opt (Cu2+ ) value depends on the site symmetry and the spin state, it is impossible to calculate opt (O) values of lattice, ␣ and  oxygen. It can only be said that the energy necessary to transfer electrons from oxygen to Cu2+ follows the order of  oxygen <␣ oxygen < lattice oxygen. In the d–d transition region (λ>500 nm), only the A4 band was observed after the standard pre-treatment (1). The d–d transition bands were slightly modified after the [Ox, 100→RT]p treatment (2), indicating that the adsorption of ␣ oxygen have little influence on the local site symmetry of Cu2+ ions. On the contrary, the treatment of [Ox, 500→RT]p (3) caused the A4 band to decrease and the A5 band to appear. The thermal behaviors of the bands were examined by evacuating the sample after the [Ox, 500→RT]p treatment at elevated temperatures. The band intensities of A4 and A5
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bands were almost unchanged below 300◦ C. When the sample was evacuated at 400◦ C, where the desorption of  oxygen and therefore the decrease of the A3 band due to the LMCT of  oxygen→Cu2+ took place, the A4 and A5 bands increased and deceased, respectively. After the evacuation at 500◦ C, the spectrum almost coincided with that after the standard pre-treatment (1). These results clearly indicate that the adsorption of  oxygen brings about a decrease of Cu2+ ions giving the A4 band and an increase of Cu2+ ions giving the A5 band. It is also implied that the assignment of A4 and A5 bands gives useful information about the real active center for the NO decomposition because  oxygen has strong relation to the active center as described above. Several references [29,32,33] indicate that the A5 band is assignable to Cu2+ in the tetrahedral symmetry. The assignment of the A4 bands, on the other hand, is not straightforward, because Cu2+ ions in various symmetries such as octahedrons with and without distortion and square pyramid give the d–d transitions in the range of 600–900 nm [29,32]. It can be seen that the A4 band is composed of at least two peaks at 650 and 750–800 nm. Based on the assignment of Praliaud et al. [29], the former might originate from Cu2+ in distorted octahedral (nearly planer) configuration and the latter from Cu2+ in nearly perfect octahedral configuration. If this is true, then it follows that the adsorption of  oxygen occurs in such a way that Cu2+ ions in nearly planar and perfect octahedral symmetries decrease while Cu2+ ions in tetrahedral symmetry increase. This seems to be realized only when Cu2+ ions, which are coordinated by six oxygen in distorted and perfect octahedral configurations, migrate to a more open site at which the Cu2+ can take the tetragonal configuration after adsorbing  oxygen (atomic ELO). This would not be the case, however, if one takes into account that the bands of tetrahedral complexes are at least two-orders of magnitude more intense than those of octahedral complexes [34]; the magnitude of the change of the tetrahedral A5 band was smaller than that of the A4 band. More reasonable assignment of the A4 band should be the d–d transition of Cu2+ in trigonal (C3v ) symmetry (Fig. 9) as reported in Cu-A zeolite [35]. According to Strome and Kiler [35], the d–d transitions of Cu2+ in the trigonal configuration and with the Jahn–Teller distortion occur at 667, 800 and 926 nm, which are in the range of the A4 band. It is reported that one
Fig. 9. A model of ‘Cu2+ O3 ’ complex in trigonal symmetry (C3v ).
of the exchangeable cation sites in MFI, which is on the hexagonal rings facing the channels, has the C3v site symmetry [36]. It seems that finding three oxygen atoms for the trigonal symmetry might be easier for a Cu2+ ion than finding four-, five- or six-coordination sites. The reason why Cu2+ in the trigonal configuration have not been so far observed by EPR might be because DRS is more sensitive to the local site symmetry than EPR. If one accepts that the A4 band originates from Cu2+ in the trigonal configuration, the events of the ␣and -oxygen adsorption can be described as follows. The adsorption of oxygen naturally causes the oxidation of Cu+ into Cu2+ . The adsorption of  oxygen accompanies the change of the symmetry of Cu2+ from three-coordinated trigonal to four-coordinated tetrahedral configurations. The most reasonable explanation is that the adsorption of  oxygen takes place so as to bridge Cu2+ in trigonal symmetry and Cu+ . The adsorption of ␣ oxygen, on the other hand, brings about little change of the configuration of Cu2+ , suggesting that the adsorption of ␣ oxygen do not involve Cu2+ and therefore ␣ oxygen adsorbs on isolated Cu+ . As stated above, the active center for the direct NO decomposition is created after the desorption of  oxygen. Accordingly, it can be concluded that the active center contains both Cu2+ and Cu+ ions (Cu+ –Cu2+ pair). It is natural that at least two such sites are necessary to catalyze the NO decomposition reaction. It is not clear at present whether the real active site is two Cu+ –Cu2+ pairs in close proximity or multinuclear (oligomer) type containing Cu+ and Cu2+ . Recently, Pârvulescu et al. [37] proposed the asymmetric active centers composed of Cu+ and Cu2+ , although they have ELO-bridged structure like biatomic (Cu+ –O–Cu2+ ) or tetraatomic (Cu+ –O–Cu2+ )2 species. More consideration of the interaction with NO and the active site is of course necessary, but we note that Centi and Perathoner [11] proposed a mechanism of NO decomposition over Cu-MFI in which both Cu+ and Cu2+ take part.
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Fig. 10. Transient behavior of NO decomposition activity at 500◦ C and intensities of PL and DS bands originating from Cu ions (see text for the details of the experimental procedure). Catalyst: Mg(35)–Cu(65)-MFI. L(480), L(540): Cu+ emission intensity of 480 and 540 nm bands. F(R∞ ) [A4 ]: intensity of A4 DR band.
3.4. Transient behavior of Cu-MFI from fresh to steady states As stated above, the exiting states of Cu ions in Cu-MFI are different between fresh and steady-state catalysts. The transient behavior in approaching the steady state was pursued by semi-simultaneous measurements of the catalytic activity, DR and PL spectroscopy. Fig. 10 shows the results with Mg(35)–Cu(65)-MFI which was prepared by the successive ion-exchange with aqueous Mg(NO3 )2 and then with aqueous Cu(CH3 COO)2 [37]. In this experiment a special glass-tube reactor, which had a branched tube with a quartz cell for spectroscopic measurements was used. The catalyst was first placed in the reaction zone and treated in a helium stream at 500◦ C for 1 h, followed by cooling down to RT and disconnection of the reactor tube from the main flow reactor. The catalyst was then transferred into the quartz cell and DR and PL spectra were recorded (t=0). After the catalyst was re-transferred to the reactor zone, the reaction tube was connected to the flow reactor and the reaction gas, NO(0.5%)/He, was fed over the catalyst at 500◦ C. The reactor tube was separated from the flow reactor system after passing the reaction gas for a prescribed period, and DR and PL spectra were recorded at RT. The activity measurement and spectrum acquisition were repeated up to 240 min.
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Fig. 10 clearly shows that the transient behavior was observed up to about 75 min. The conversion of NO into N2 was high for the first 45 min and reached the steady state after about 75 min had passed. The emanation of O2 in the gas phase was observed after about 90 min. At the beginning (t<5 min), strong Cu+ emission was observed, while the A4 band of DRS was very low. High NO decomposition activity in this region shows that PL-active Cu+ ions are active for the NO decomposition. As the reaction time passed, the Cu+ emission progressively decreased and became almost invisible after 75 min. On the contrary, the A4 DR band increased and reached a steady value after 75 min. The steady states for both the activity and the spectroscopic behavior of Cu ions continued till the end of the experiment (240 min). Because the catalyst was cooled down in the presence of NO (t>0 min) and O2 (t>100 min) in this experiment, the adsorption of them and the decomposition of NO would take place during the cooling process. After finishing the experiment, the catalyst was heated in a stream of He at 500◦ C and the PL and DR spectra were measured, but the spectra were identical before and after the He treatment at 500◦ C. In addition, the appearance of the A3 band (LMCT of  oxygen → Cu2+ ) and the decrease in the A4 -band intensity, which are indicative of the  oxygen formation, were never observed. These results indicate that, even if adsorption of NO and O2 and the decomposition of NO take place during cooling, they had no relation to the PL and DR spectra at least after reaching the steady state. In other words, states of Cu ions observed by PL and DR spectroscopy reflect the working state. The complementary behaviors of PL (Cu+ ) and DR (Cu2+ in trigonal symmetry) intensities in the transient region, coupled with the accumulation of oxygen, indicate that abundant Cu+ ions which formed by treating the fresh sample in He at 500◦ C are oxidized to Cu2+ by oxygen resulting from decomposed NO and that some Cu2+ thus formed, probably a small portion of them, sit on the trigonal sites to be a part of the active centers under working state.
4. Conclusions The redox behavior of Cu ions in the MFI zeolite has been investigated by means of TPD of oxygen, PL and DR spectroscopy, and the catalytic activity for
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the direct decomposition of NO has been evaluated by steady state and pulse reactions. By taking account of characterization and activity results, we have proposed here that the real active center for the NO decomposition catalysis contains both Cu+ and Cu2+ ions. References [1] M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya, S. Kagawa, J. Chem. Soc., Chem. Commun. (1986) 1272. [2] M. Iwamoto, H. Furukawa, S. Kagawa, Stud. Surf. Sci. Catal. 28 (1986) 943. [3] M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett. (1989) 213. [4] M. Iwamoto, H. Yahiro, Y. Yu-u, S. Shundo, N. Mizuno, Shokubai (Catalyst) 32 (1990) 430. [5] W. Held, A. König, T. Richter, L. Puppe, SAE Paper (1990) 900496. [6] M. Iwamoto, S. Hamada, Catal. Today 10 (1991) 57. [7] M. Iwamoto, N. Mizuno, H. Yahiro, Sekiyu Gakkaishi 34 (1991) 375. [8] M. Iwamoto, H. Yahiro, Catal. Today 22 (1994) 5. [9] K.C.C. Kharas, D.J. Liu, H.J. Robota, Catal. Today 26 (1995) 129. [10] M. Shelef, Chem. Rev. 95 (1995) 209. [11] G. Centi, S. Perathoner, Appl. Catal. A 132 (1995) 179. [12] A. Fritz, V. Pitchon, Appl. Catal. B 13 (1997) 1. [13] Y. Li, W.K. Hall, J. Phys. Chem. 94 (1990) 6145. [14] Y. Li, W.K. Hall, J. Catal. 129 (1991) 202. [15] M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, S. Kagawa, J. Phys. Chem. 95 (1991) 3727. [16] M. Iwamoto, H. Yahiro, N. Mizuno, W.-X. Zhang, Y. Mine, H. Furukawa, S. Kagawa, J. Phys. Chem. 96 (1992) 9360. [17] M. Shelef, Catal. Lett. 15 (1992) 305.
[18] Y. Teraoka, C. Tai, H. Furukawa, S. Kagawa, K. Asakura, Y. Iwasawa, Shokubai (Catalyst) 32 (1990) 426. [19] J. Valyon, W.K. Hall, J. Catal. 143 (1993) 520. [20] K. Eränen, N. Kumar, L.-E. Lindfors, Appl. Catal. B 4 (1994) 213. [21] Z. Wang, A.V. Sklyarov, G.W. Keulks, Catal. Today 33 (1997) 291. [22] R. Beer, G. Calzaferri, I. Kamber, J. Chem. Soc., Chem. Commun. (1991) 1489. [23] J. Dedecek, B. Wichtelová, J. Phys. Chem. 98 (1994) 5721. [24] M. Anpo, M. Matsuoka, Y. Shioya, H. Yamashita, E. Giamello, C. Morterra, M. Che, H.H. Patterson, S. Webber, S. Ouellette, M.A. Fox, J. Phys. Chem. 98 (1994) 5744. [25] B. Wichtelová, J. Dedecek, Z. Sobal´ık, A. Vondrová, K. Klier, J. Catal. 169 (1997) 194. [26] J. Dedecek, B. Wichtelová, P. Kubát, Microporous and Mesoporous Mater. 32 (1999) 63. [27] R.A. Schoonheydt, Catal. Rev. 35 (1993) 129. [28] Y. Itho, S. Nishiyama, S. Tsuruya, M. Masai, J. Phys. Chem. 98 (1994) 960. [29] H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B 16 (1998) 359. [30] E.D. Garbowski, C. Mirodatos, J. Phys. Chem. 86 (1982) 97. [31] D.H. Strome, K. Kiler, J. Phys. Chem. 84 (1980) 981. [32] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd Edition, Elsevier, Amsterdam, 1984. [33] R. Hierl, H. Knözinger, H.-P. Urbach, J. Catal. 69 (1981) 475. [34] R.A. Schoonheydt, in: F. Delannay (Ed.), Characterization of Heterogeneous Catalysts, Marcel Dekker, New York, 1984, p. 142. [35] D.H. Strome, K. Kiler, in: J. Koter (Ed.), Molecular Sieve II, ACS Symp. Series, Vol. 135, 1977, p. 108. [36] G. Spoto, A. Zecchina, S. Bordiga, G. Ricchiardi, G. Marta, G. Leofanti, G. Petrini, Appl. Catal. B 3 (1994) 151. [37] S. Kagawa, H. Ogawa, H. Furukawa, Y. Teraoka, Chem. Lett. (1991) 407.
Characterization and NO decomposition activity of Cu-MFI zeolite in relation to redox behavior Y. Teraoka∗ , C. Tai, H. Ogawa1 , H. Furukawa, S. Kagawa Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852-8521, Japan Received 24 February 2000; received in revised form 10 April 2000; accepted 11 April 2000
Abstract The redox behavior and states of Cu ions in Cu ion-exchanged MFI (Cu(n)-MFI, n: exchange level) have been investigated by means of temperature-programmed desorption (TPD) of oxygen, diffuse reflectance (DR) UV–VIS spectroscopy and Cu+ photoluminescence (PL) spectroscopy. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance of three desorption peaks: ␣ (below 200◦ C),  (300–500◦ C) and ␥ (above 500◦ C). It has been suggested that ␣ and  oxygen are extra-lattice oxygen adsorbed on Cu ions, while ␥ oxygen is lattice oxygen coordinated to Cu ions. The Cu+ emission was tremendously reduced once the catalyst contacted with O2 and NO at elevated temperatures such as 500◦ C, and it was almost invisible under the working state of the catalyst, suggesting that PL-active Cu+ ions are not real active sites under the working state. The desorption of  oxygen was intimately related to the creation of active centers for the NO decomposition reaction. DR measurements showed that the desorption of  oxygen caused tetragonal Cu2+ to decrease and trigonal Cu2+ to increase simultaneously. It has been proposed that both Cu2+ and Cu+ are involved in the NO decomposition catalysis over Cu-MFI under the working state. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Cu-MFI zeolite; Nitric oxide decomposition; Redox behavior; Active site
1. Introduction Iwamoto et al. first reported the high and stationary activity of Cu ion-exchanged MFI zeolite (Cu-MFI) for the direct decomposition of NO into N2 and O2 [1–3]. These pioneering works, coupled with the discovery of the selective reduction of NOx with hydrocarbons [4,5], have triggered a huge number of related studies, and have contributed to the establishment of a field of NOx catalysis with Cu-loaded zeolites. Apart ∗ Corresponding author. Tel.: +81-95-848-9652; fax: +81-95-848-9652. E-mail address: [email protected] (Y. Teraoka). 1 Present address: Nanyo Research Laboratory, Tosoh Corporation, Shinnanyo 746-8501, Japan.
from original papers, many review articles on this subject have been published [6–12]. There are many common features of the catalytic behaviors of Cu-MFI for the direct NO decomposition. The reaction is a true decomposition [13,14]; it passes through a maximum around 500◦ C; and it is inhibited by oxygen. Copper is the most active or practically the only ingredient in zeolitic NO decomposition catalysts, and the specific activity of exchanged Cu ion increases with decreasing the Al content of host zeolites [2]. The activity increases with an increase in the Cu content, and over-exchanged catalysts are more active than under-exchanged catalysts [3,15]. Reaction mechanism, intermediates and active Cu site/species concerning the NO decomposition over Cu-MFI have been extensively investigated
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by such techniques as IR, EPR, XPS, XAFS, Cu+ phosphorescence, diffuse reflectance UV–VIS and temperature-programmed methods. The details will be left to review articles [6–12], but the real active/working states of Cu-MFI catalysts are still controvertible: for example, some contradictory reaction mechanisms were reported, such as redox mechanism with binuclear active center (a pair of Cu+ ins or [Cu–O–Cu]) [2,14,16] and non-redox mechanism with isolated Cu2+ ion [17]. We have previously proposed that both Cu2+ and Cu+ are involved in the NO decomposition catalysis over Cu-MFI catalysts [18]. In this paper, the redox behavior of Cu ions in the MFI zeolite under various treatments is investigated by means of temperature-programmed desorption (TPD) of oxygen, photoluminescence and diffuse reflectance UV–VIS spectroscopy. The relation between the redox behavior and the NO decomposition catalysis is also discussed.
2. Experimental 2.1. Catalysts Na-MFI (SiO2 /Al2 O3 =23.3, Tosho Corporation) was treated with 0.1 M aqueous NaNO3 overnight at 60◦ C, followed by filtration and a conventional ion-exchange procedure using aqueous Cu(II) acetate at the same temperature [3]. After the ion exchanges, samples were filtered, washed thoroughly with deionized water, and dried in an oven at 110◦ C. The samples thus obtained were referred to as Cu(n)-MFI; n in parentheses means the content of Cu in terms of ion exchange level, which was determined by atomic absorption spectroscopy after dissolving in an aqueous HF solution: the stoichiometry of Cu/Al=0.5 corresponds to 100% exchange level. Samples were treated in various ways before subjecting them to characterization experiments. The as-synthesized (fresh) samples were evacuated, exposed to O2 (100 Torr) and re-evacuated at 500◦ C (0.5 h for each treatment); this is designated the standard pre-treatment. Two types of oxygen adsorption treatment were employed; after exposure to O2 (usually 100 Torr) for 0.5 h at T1 ◦ C, samples were evacuated at the same temperature ([Ox, T1 ]) or cooled in the O2 atmosphere to T2 ◦ C ([Ox, T1 →T2 ]). The
samples were also subjected to the evacuation treatment at T◦ C ([Evac, T]). If necessary, the starting samples are indicated by subscript ‘p’ and ‘f’ for the samples after the standard pre-treatment and the fresh sample, respectively; for example, [Evac, 800]p and [Evac, 800]f mean the evacuation treatments at 800◦ C of pre-treated and fresh samples, respectively. 2.2. NO decomposition reaction The steady-state NO decomposition reaction was carried out in a fixed-bed flow reactor. The reaction gas (1 vol.% NO in He) was fed at a rate of 15 cm3 min−1 over 1.0 g catalyst which was pre-treated at 500◦ C for 1 h in a helium stream. The temperature dependence of the NO decomposition reaction over Cu(130)-MFI is depicted in Fig. 1 as a representative example. The pulse decomposition of NO was performed as follows. Catalysts (0.05 g) were packed in a reactor made of stainless steel tubing, and under He flowing at a rate of 20 cm3 min−1 a small portion of NO gas (3.8 mol) was pulsed. The reaction products of both steady-state and pulse reactions were analyzed by gas chromatography (TCD). 2.3. Characterization Temperature-programmed desorption (TPD) of oxygen was carried out in a stream of He at a heating rate of 10◦ C min−1 , and the desorbed oxygen was
Fig. 1. Temperature dependence of NO decomposition reaction over Cu(130)-MFI. NO(1%)–He(balance). W/F=4.0 g s cm−3 .
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monitored by a TCD detector. When an equimolar mixture of 16 O2 and 18 O2 was used as the adsorption gas, a quadrupole mass spectrometer (QMS, Nichiden Varian TE-600) was used to monitor desorbed gaseous components. Diffuse reflectance (DR) UV–VIS spectra were recorded at room temperature (RT) on a Shimadzu UV-3100 spectrometer equipped with an integrating sphere using BaSO4 as a reference material; the data were processed according to Kubelka–Munk theory. The photoluminescence (PL) spectra were measured at RT employing a Nihonbunko FP770 spectrometer.
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Fig. 3. Desorption amounts of ␣,  and ␥ oxygen per Cu ion from Cu(n)-MFI as a function of Cu ion-exchange level.
3. Results and discussion 3.1. Oxygen desorption behavior TPD chromatograms of oxygen from Cu(n)-MFI are shown in Fig. 2; oxygen adsorption was performed by cooling the Cu(n)-MFI in O2 (100 Torr) from 500◦ C to RT after the standard pre-treatment at 500◦ C, [Ox, 500→RT]p . The amount of oxygen desorbed was a trace for Na-MFI (n=0) and increased with an increase in the Cu content n, indicating that Cu ions are responsible for the emergence of the oxygen desorption. TPD chromatograms of oxygen from Cu(n)-MFI were characterized by the appearance, in principle, of three desoprtion peaks, ␣,  and ␥. As can be recognized
Fig. 2. TPD chromatograms of oxygen from Cu(n)-MFI (n=0, 35, 48, 78 and 130). Oxygen adsorption, [Ox, 500→RT]p .
from the desorption chromatograms of Cu(78)-MFI, ␣ peak below 200◦ C seems to consist of two peaks, but it is tentatively considered to be one desorption peak in this study. With an increase in the Cu content, the amount of oxygen desorbed under  and ␥ peaks increased, and the onset and peak temperatures of  and ␥ peaks decreased and increased, respectively. For the samples with larger n (78, 130%), ␣ peak was additionally observed below 200◦ C. The oxygen desorption behavior observed in this study is essentially consistent with that so far reported [19–21]. In Fig. 3, the amounts of each desorbed species per Cu ion (specific amount) are plotted as a function of the ion-exchange level. The total amount of oxygen desorbed up to 750◦ C corresponded to 0.08 (n=35), 0.09 (48), 0.13 (78) and 0.15 (130) O2 /Cu, indicating that oxygen desorption/adsorption capacity per Cu ion increases with increasing the exchange level of Cu. The specific amount of ␥ oxygen was almost constant irrespective of the ion-exchange level. Judging from the fact that the amount of ␥ oxygen depended almost exclusively on the amount of Cu ions loaded, one may conclude that ␥ oxygen is lattice oxygen coordinated to Cu ions, as also suggested by Valyon and Hall [19]. The specific amounts of ␣ and  oxygen, on the other hand, depended significantly on the ion-exchange level of Cu ions, and they must be the adsorbed species on Cu ions or the extra-lattice oxygen (ELO) [19–21]. In the following, the nature of ␣ and  oxygen was investigated in more detail. Fig. 4 depicts TPD chromatograms of oxygen from Cu(130)-MFI below 550◦ C after the treatments of [Ox, 500→RT]p (1), [Ox, 100→RT]p (2) and [Ox, 300]p
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Fig. 4. TPD chromatograms of oxygen from Cu(130)-MFI. Oxygen adsorption: (1) [Ox, 500→RT]p , (2) [Ox, 100→RT]p , (3) [Ox, 300]p .
(3). As described above, the desorption of ␣ and  oxygen was separated below 200◦ C and above 300◦ C, respectively, with the [Ox, 500→RT]p treatment (1). It can be seen that selective formation of ␣ and  oxygen was realized by the oxygen adsorption treatments of [Ox, 100→RT]p (2) and [Ox, 300]p (3), respectively. The amount of ␣ oxygen after the [Ox, 100→RT]p treatment (in the absence of  oxygen) was far larger than that after the [Ox, 500→RT]p treatment (in the presence of  oxygen). This strongly suggests that Cu ions in the MFI zeolite can adsorb both ␣ and  oxygen depending on the oxygen adsorption treatment; in other words, there are no specific Cu sites adsorbing ␣ or  oxygen. TPD experiments were also carried out using an equimolar mixture of 16 O2 and 18 O2 after the selective formation of ␣ ([Ox, 100→RT]p ) and  ([Ox, 300]p ) oxygen. In both cases, the main desorbed species was 16 O18 O, and the compositions of desorbed O2 , [16 O18 O]2 /[16 O2 ][18 O2 ], were close to 4 in the temperature ranges of both ␣ and  desorption, indicating that the homomolecular exchange of gaseous molecules reached the equilibrium. The 18 O fraction in the temperature range of the ␣ desorption was almost constant (0.50–0.48) and close to that of the charged gaseous mixture (0.50), while that of the  desorption was smaller than 0.50 and decreased with increasing temperature (0.40 at 300–350◦ C, 0.25 at 400◦ C, 0.14 at 450◦ C). From these results, it can be concluded
that  oxygen exchangeable with lattice oxygen is a dissociatively adsorbed species, and this must be the atomic ELO bridging two Cu ions [16,19]. The nature or real form of ␣ oxygen, which is not exchangeable with lattice oxygen but can contribute to the equilibration of the homomolecular exchange reaction, is not clear at present. Valyon and Hall assumed that the low-temperature species (␣ oxygen) was molecularly adsorbed oxygen [19]. It might be also possible to assume that ␣ oxygen is atomic species because the homomolecular exchange reaction generally proceeds with the participation of atomic adsorbed species; the temperature is too low for ␣ oxygen to exchange with the lattice oxygen. As stated above, the ␣ peak is composed of two peaks, and therefore one might correspond to the molecular species and the other to the atomic species. Since ␣ oxygen is not so important for the creation of the real active centers for NO decomposition reaction (vide infra), no further investigation was carried out on the nature of ␣ oxygen. The adsorption site of ␣ and  oxygen is the same as stated above, but the adsorption state and binding energy are different from each other because of the different desorption temperatures. The  oxygen bridges two Cu ions, while we consider, at present, that an ␣ oxygen is adsorbed on one Cu ion. The formation of the bridging ELO ( oxygen) would require the migration and access of Cu ions. When oxygen is adsorbed at 300◦ C or by cooling from 500◦ C to RT,  oxygen is preferentially formed because the temperature is enough high for Cu ions to migrate, come close to each other and to be bridged by the atomic ELO. In oxygen adsorption at lower temperatures such as [Ox, 100→RT], Cu ions can no longer migrate, so that oxygen is adsorbed on one (isolated) Cu ion. If ␣ oxygen is a molecularly adsorbed species, one additional possibility is that the temperature is too low for the oxygen molecule to be dissociated. When the Cu ion-exchange level became high enough, isolated Cu ions might be left after adsorbing  oxygen and they might be the adsorption sites of ␣ oxygen. This would be the case for Cu(78)- and Cu(130)-MFI in Fig. 2. 3.2. Relationship between oxygen desorption behavior and NO decomposition activity Comparison between Figs. 1 and 2 shows that the take-off temperature of the NO decomposition
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Fig. 5. Relation between the amount of  oxygen desorbed and the NO decomposition activity at 450◦ C. Catalysts, Cu(n)-MFI. Numerals in the figure are ion-exchange level n in Cu(n)-MFI.
into N2 and O2 is well coincident with that of the  desorption. It was reported that the NO decomposition activity of Cu-MFI showed the ‘S-shaped’ dependence on the exchange level of Cu ions [15]. When the NO decomposition activity was plotted against the amount of  oxygen (Fig. 5), on the other hand, an almost linear relation was observed between the two quantities in the range covering under- and over-exchanged levels. These results strongly imply, though indirectly, that Cu ions/sites carrying  oxygen should be the active center for the direct NO decomposition when they release  oxygen. In order to confirm this, the pulse NO decomposition was carried out using Cu(76)-MFI catalyst. Fig. 6(A) and (B) show the results at 300◦ C at which both N2 and N2 O are formed in the steady-state reaction (Fig. 1)
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and the desorption of  oxygen hardly takes place (Fig. 2). The catalyst in Fig. 6 (A) was treated by [Ox, 500→300]p and therefore carried  oxygen, while the catalyst free from  oxygen, Fig. 6(B), was obtained by heating in a helium stream at 500◦ C for 0.5 h before subjecting the pulse reaction. Over the catalyst with  oxygen (Fig. 6(A)), the formation of N2 was never detected, and that of N2 O reached the steady value after the fourth pulse. In case of the catalyst without  oxygen (Fig. 6(B)), on the other hand, the formation of N2 was observed at the first three pulses; the amount of N2 formed progressively decreased with increasing the pulse number and none was detected after the fourth pulse. With increasing the pulse number, the N2 O formation reached a maximum at the second pulse, decreased progressively (second to sixth pulses) and eventually reached the steady value equal to that in Fig. 6(A) after the sixth pulse. After the tenth pulse of NO, the catalyst was cooled down to RT and the desorption of gaseous molecules was monitored by QMS while heating in the He stream at 10◦ C min−1 . Oxygen was a sole desorbed species in the range between RT and 550◦ C, and the desorption temperature (>300◦ C) completely agreed with that of  oxygen. The sum of NO converted into N2 in Fig. 6(B), 2.4 mol, was reasonably close to the amount of  oxygen, 2.6 mol, calculated for 0.05 g of Cu(76)-MFI charged in the pulse reactor. These results indicate that oxygen atoms resulting from the direct NO decomposition into N2 accumulate in the catalyst as  oxygen. Accordingly, the catalyst after the fourth pulse in Fig. 6(B) is expected to carry  oxygen to its capacity. This is supported by the
Fig. 6. Pulse decomposition of NO over Cu(76)-MFI at 300◦ C (A,B) and 500◦ C (C). Pre-treatment: (A) [Ox, 500→300]p , (B) He-purge at 500◦ C for 0.5 h, (C) standard pre-treatment.
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resemblance of the pulse reaction results of Fig. 6(B) after the fourth pulse and Fig. 6(A) with respect to the dependence on the pulse number and the reaction products. When the pulse reaction was carried out at 500◦ C (Fig. 6(C)), only the formation of N2 was observed at higher conversion level. This is reasonable, because at 500◦ C the conversion of NO into N2 O is never observed in the steady-state reaction (Fig. 1) and the spontaneous desorption of  oxygen is expected to proceed under the He stream between the NO pulses. These results confirm the strong implication of  oxygen or Cu ions carrying  oxygen to the direct NO decomposition catalysis. When  oxygen desorbs spontaneously or thermally, the resulting site becomes the active center of the NO decomposition reaction into N2 and O2 . Considering that  oxygen is bridging two Cu ions, at least two Cu ions are necessary to form the active center (see below for more detail). The pulse reaction results also show that the active site for the N2 O formation is different from that for the N2 formation at 300◦ C. 3.3. Spectroscopic characterization Since Iwamoto et al. pointed out the importance of Cu+ ions and the redox change between Cu+ and Cu2+ for the appearance of the stationary NO decomposition activity [2,16], photoluminescence (PL) technique has often been used to detect Cu+ in zeolite matrices [7,22–26]. Fig. 7 shows the Cu+ emission spectra of Cu(83)-MFI after various treatments. When the fresh catalyst was evacuated at various temperatures, [Evac, T]f , no Cu+ emission was observed below 300◦ C, a less resolved spectrum at 400◦ C (Fig. 7(A)) and a well-resolved spectrum with maxima at 480 and 540 nm above 500◦ C (B, C). The bands at 480 and 540 nm were assigned to Cu+ in close proximity to the two and one framework Al, respectively [23]. With an increase in the evacuation temperature, the intensity of the 540 nm band passed a maximum at 600◦ C, while that of the 480 nm band leveled off above 600◦ C. The dependence of the Cu+ emission on the ion-exchange level (n) was examined with Cu(n)-MFI (n≥35) after the [Evac, 600]f treatment. The 480 nm band was observed even with Cu(35)-MFI and the intensity increased moderately with increasing n. The 540 nm band intensity, on the other hand, was vary
weak at n<50 and increased tremendously with increasing n. The 480 and 540 nm bands predominated at ion-exchange level below and above ca. 80%, respectively. The dependencies of the Cu+ emission on the evacuation temperature and the Cu content were essentially consistent with those reported [7,23]. It should be noted here that the strong Cu+ emission was observed only when the as-prepared (fresh) Cu-MFI was evacuated at higher temperatures and that the Cu+ emission tremendously weakened once the Cu-MFI was contacted with oxidizing gases such as O2 and NO at elevated temperatures like 500◦ C. As seen from spectrum D in Fig. 7, the Cu+ emission almost disappeared after the standard pre-treatment at 500◦ C, that is, the evacuation, oxidation and re-evacuation at 500◦ C. Judging from the TPD result described in Section 3.1, one would expect that a substantial amount of Cu+ is produced when the pre-treated sample is evacuated at 800◦ C; the desorbed oxygen should amount to more than 2.5 O/Cu. As can be seen from Fig. 7E, however, the Cu+ emission intensity was very low. This indicates that under the working state the population of PL-active Cu+ ions is low and thus the photoluminescence study gives rather little information about the steady or working state of Cu-MFI, although Cu+ ions detected by PL spectroscopy are active for the NO decomposition and should be the precursor of the active center for NOx abatement catalysis (vide infra). EPR and DRS are known to be powerful tools to investigate the existing state of Cu2+ ions [27], but the
Fig. 7. Cu+ emission spectra of Cu(83)-MFI (Ex=280 nm). Pre-treatment: (A) [Evac, 400]f , (B) [Evac, 500]f , (C) [Evac, 800]f , (D) standard pre-treatment, (E) [Evac, 800]p .
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Fig. 8. Diffuse reflectance UV–VIS spectra of Cu(130)-MFI. Pre-treatment: (1) standard pre-treatment, (2) [Ox, 100→RT]p , (3) [Ox, 500→RT]p .
application of DRS to Cu-MFI has been very limited as compared with uses of EPR [28,29]. Fig. 8 depicts DR spectra of Cu(130)-MFI after various treatments. As can be recognized from the TPD results, both ␣ and  adsorbed oxygen are absent after the standard pre-treatment (1), the [Ox, 100→RT]p treatment (2) results in the selective formation of ␣ oxygen, and the [Ox, 500→RT]p treatment (3) gives both  (major) and ␣ (minor) oxygen. Five absorption bands were observed depending on the treatments: A1 at ca. 240 nm, A2 at ca. 320 nm, A3 at ca. 440 nm, A4 between 600 and 900 nm and A5 at ca. 1640 nm. The A4 band consisted of several peaks, but here they are collectively termed the A4 band (see below for the detail). Only the A1 band was observed with Na-MFI which might originate from the framework Al–O units [30], but its intensity, F(R∞ )=0.5 at the peak maximum, was very weak as compared with that of Cu-MFI. In addition, since the 3d10 →3d9 4 s1 transition band of Cu+ , which is used for the excitation of Cu+ photoluminescence, appears around 280 nm [30,31], the contribution of Cu+ , if present, to the A1 and A2 bands should be taken into account. However, this effect might be negligible in Fig. 8 because all the samples experienced the standard pre-treatment at 500◦ C, which causes the tremendous decrease in the Cu+ emission (Fig. 7) and excitation (not shown). Accordingly, five bands observed should be related to Cu2+ ions. As stated below, A1 , A2 and A3 bands are the ligand-to-metal
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charge transfer (LMCT) transition from oxygen to Cu2+ , while A4 and A5 are the d–d transition of Cu2+ . It is known that the LMCT of O→Cu2+ occurs in the range of 230–255 nm for Cu ion-exchanged zeolites, which includes all the possible LMCT transitions irrespective of the symmetry of the Cu2+ complex [27]. Since Cu(130)-MFI after the standard pre-treatment (1) is considered to carry no ELO (neither ␣ nor  oxygen), most of the A1 band should be contributed by the LMCT from lattice oxygen to Cu2+ ion. Actually, the A1 -band intensity of Cu(n)-MFI after the standard pre-treatment increased with increasing n. The A2 band appeared after the [Ox, 100→RT]p treatment (2) and the A3 band by the [Ox, 500→RT]p treatment (3), strongly suggesting that the A2 and A3 bands have some relation to ␣ and  oxygen, respectively. Since d–d transitions of Cu2+ occur above 500 nm, the A2 and A3 bands could be assignable to LMCT bands from ␣ oxygen and  oxygen to Cu2+ ion, respectively. The increase of the A1 -band intensity by the oxygen adsorption treatment as well as the order of the intensity: 1<2<3 may reflect the fact that oxygen adsorption causes an increase in the Cu2+ concentration by the oxidation of Cu+ to Cu2+ . The position of the LMCT band maximum (ν in unit of cm−1 ) can be related to the difference in optical electronegativity of the oxygen and Cu2+ , χ opt (O) and opt (Cu2+ ) [32]. ν (cm−1 ) = 30000[χopt (O) − χopt (Cu+2 )]
(1)
Since the opt (Cu2+ ) value depends on the site symmetry and the spin state, it is impossible to calculate opt (O) values of lattice, ␣ and  oxygen. It can only be said that the energy necessary to transfer electrons from oxygen to Cu2+ follows the order of  oxygen <␣ oxygen < lattice oxygen. In the d–d transition region (λ>500 nm), only the A4 band was observed after the standard pre-treatment (1). The d–d transition bands were slightly modified after the [Ox, 100→RT]p treatment (2), indicating that the adsorption of ␣ oxygen have little influence on the local site symmetry of Cu2+ ions. On the contrary, the treatment of [Ox, 500→RT]p (3) caused the A4 band to decrease and the A5 band to appear. The thermal behaviors of the bands were examined by evacuating the sample after the [Ox, 500→RT]p treatment at elevated temperatures. The band intensities of A4 and A5
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bands were almost unchanged below 300◦ C. When the sample was evacuated at 400◦ C, where the desorption of  oxygen and therefore the decrease of the A3 band due to the LMCT of  oxygen→Cu2+ took place, the A4 and A5 bands increased and deceased, respectively. After the evacuation at 500◦ C, the spectrum almost coincided with that after the standard pre-treatment (1). These results clearly indicate that the adsorption of  oxygen brings about a decrease of Cu2+ ions giving the A4 band and an increase of Cu2+ ions giving the A5 band. It is also implied that the assignment of A4 and A5 bands gives useful information about the real active center for the NO decomposition because  oxygen has strong relation to the active center as described above. Several references [29,32,33] indicate that the A5 band is assignable to Cu2+ in the tetrahedral symmetry. The assignment of the A4 bands, on the other hand, is not straightforward, because Cu2+ ions in various symmetries such as octahedrons with and without distortion and square pyramid give the d–d transitions in the range of 600–900 nm [29,32]. It can be seen that the A4 band is composed of at least two peaks at 650 and 750–800 nm. Based on the assignment of Praliaud et al. [29], the former might originate from Cu2+ in distorted octahedral (nearly planer) configuration and the latter from Cu2+ in nearly perfect octahedral configuration. If this is true, then it follows that the adsorption of  oxygen occurs in such a way that Cu2+ ions in nearly planar and perfect octahedral symmetries decrease while Cu2+ ions in tetrahedral symmetry increase. This seems to be realized only when Cu2+ ions, which are coordinated by six oxygen in distorted and perfect octahedral configurations, migrate to a more open site at which the Cu2+ can take the tetragonal configuration after adsorbing  oxygen (atomic ELO). This would not be the case, however, if one takes into account that the bands of tetrahedral complexes are at least two-orders of magnitude more intense than those of octahedral complexes [34]; the magnitude of the change of the tetrahedral A5 band was smaller than that of the A4 band. More reasonable assignment of the A4 band should be the d–d transition of Cu2+ in trigonal (C3v ) symmetry (Fig. 9) as reported in Cu-A zeolite [35]. According to Strome and Kiler [35], the d–d transitions of Cu2+ in the trigonal configuration and with the Jahn–Teller distortion occur at 667, 800 and 926 nm, which are in the range of the A4 band. It is reported that one
Fig. 9. A model of ‘Cu2+ O3 ’ complex in trigonal symmetry (C3v ).
of the exchangeable cation sites in MFI, which is on the hexagonal rings facing the channels, has the C3v site symmetry [36]. It seems that finding three oxygen atoms for the trigonal symmetry might be easier for a Cu2+ ion than finding four-, five- or six-coordination sites. The reason why Cu2+ in the trigonal configuration have not been so far observed by EPR might be because DRS is more sensitive to the local site symmetry than EPR. If one accepts that the A4 band originates from Cu2+ in the trigonal configuration, the events of the ␣and -oxygen adsorption can be described as follows. The adsorption of oxygen naturally causes the oxidation of Cu+ into Cu2+ . The adsorption of  oxygen accompanies the change of the symmetry of Cu2+ from three-coordinated trigonal to four-coordinated tetrahedral configurations. The most reasonable explanation is that the adsorption of  oxygen takes place so as to bridge Cu2+ in trigonal symmetry and Cu+ . The adsorption of ␣ oxygen, on the other hand, brings about little change of the configuration of Cu2+ , suggesting that the adsorption of ␣ oxygen do not involve Cu2+ and therefore ␣ oxygen adsorbs on isolated Cu+ . As stated above, the active center for the direct NO decomposition is created after the desorption of  oxygen. Accordingly, it can be concluded that the active center contains both Cu2+ and Cu+ ions (Cu+ –Cu2+ pair). It is natural that at least two such sites are necessary to catalyze the NO decomposition reaction. It is not clear at present whether the real active site is two Cu+ –Cu2+ pairs in close proximity or multinuclear (oligomer) type containing Cu+ and Cu2+ . Recently, Pârvulescu et al. [37] proposed the asymmetric active centers composed of Cu+ and Cu2+ , although they have ELO-bridged structure like biatomic (Cu+ –O–Cu2+ ) or tetraatomic (Cu+ –O–Cu2+ )2 species. More consideration of the interaction with NO and the active site is of course necessary, but we note that Centi and Perathoner [11] proposed a mechanism of NO decomposition over Cu-MFI in which both Cu+ and Cu2+ take part.
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Fig. 10. Transient behavior of NO decomposition activity at 500◦ C and intensities of PL and DS bands originating from Cu ions (see text for the details of the experimental procedure). Catalyst: Mg(35)–Cu(65)-MFI. L(480), L(540): Cu+ emission intensity of 480 and 540 nm bands. F(R∞ ) [A4 ]: intensity of A4 DR band.
3.4. Transient behavior of Cu-MFI from fresh to steady states As stated above, the exiting states of Cu ions in Cu-MFI are different between fresh and steady-state catalysts. The transient behavior in approaching the steady state was pursued by semi-simultaneous measurements of the catalytic activity, DR and PL spectroscopy. Fig. 10 shows the results with Mg(35)–Cu(65)-MFI which was prepared by the successive ion-exchange with aqueous Mg(NO3 )2 and then with aqueous Cu(CH3 COO)2 [37]. In this experiment a special glass-tube reactor, which had a branched tube with a quartz cell for spectroscopic measurements was used. The catalyst was first placed in the reaction zone and treated in a helium stream at 500◦ C for 1 h, followed by cooling down to RT and disconnection of the reactor tube from the main flow reactor. The catalyst was then transferred into the quartz cell and DR and PL spectra were recorded (t=0). After the catalyst was re-transferred to the reactor zone, the reaction tube was connected to the flow reactor and the reaction gas, NO(0.5%)/He, was fed over the catalyst at 500◦ C. The reactor tube was separated from the flow reactor system after passing the reaction gas for a prescribed period, and DR and PL spectra were recorded at RT. The activity measurement and spectrum acquisition were repeated up to 240 min.
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Fig. 10 clearly shows that the transient behavior was observed up to about 75 min. The conversion of NO into N2 was high for the first 45 min and reached the steady state after about 75 min had passed. The emanation of O2 in the gas phase was observed after about 90 min. At the beginning (t<5 min), strong Cu+ emission was observed, while the A4 band of DRS was very low. High NO decomposition activity in this region shows that PL-active Cu+ ions are active for the NO decomposition. As the reaction time passed, the Cu+ emission progressively decreased and became almost invisible after 75 min. On the contrary, the A4 DR band increased and reached a steady value after 75 min. The steady states for both the activity and the spectroscopic behavior of Cu ions continued till the end of the experiment (240 min). Because the catalyst was cooled down in the presence of NO (t>0 min) and O2 (t>100 min) in this experiment, the adsorption of them and the decomposition of NO would take place during the cooling process. After finishing the experiment, the catalyst was heated in a stream of He at 500◦ C and the PL and DR spectra were measured, but the spectra were identical before and after the He treatment at 500◦ C. In addition, the appearance of the A3 band (LMCT of  oxygen → Cu2+ ) and the decrease in the A4 -band intensity, which are indicative of the  oxygen formation, were never observed. These results indicate that, even if adsorption of NO and O2 and the decomposition of NO take place during cooling, they had no relation to the PL and DR spectra at least after reaching the steady state. In other words, states of Cu ions observed by PL and DR spectroscopy reflect the working state. The complementary behaviors of PL (Cu+ ) and DR (Cu2+ in trigonal symmetry) intensities in the transient region, coupled with the accumulation of oxygen, indicate that abundant Cu+ ions which formed by treating the fresh sample in He at 500◦ C are oxidized to Cu2+ by oxygen resulting from decomposed NO and that some Cu2+ thus formed, probably a small portion of them, sit on the trigonal sites to be a part of the active centers under working state.
4. Conclusions The redox behavior of Cu ions in the MFI zeolite has been investigated by means of TPD of oxygen, PL and DR spectroscopy, and the catalytic activity for
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