X-ray Photoelectron Spectroscopy of Cu-ZSM-5 Zeolite - American

Chapter 6

X-ray Photoelectron Spectroscopy of Cu-ZSM-5 Zeolite L. P. Haack and M. Shelef

Downloaded by COLUMBIA UNIV on March 1, 2013 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch006

Scientific Research Laboratory, Ford Motor Company, Mail Stop 3061, P.O. Box 2053,Dearborn,MI48121-2053

The determination of the oxidation state of copper in Cu-exchanged ZSM-5 is important for understanding its role in the mechanism of decomposition and selective reduction of NO. The task is difficult using any of the common methods. Here we evaluate the use of XPS. This work confirms that prolonged exposure of the material to the X-ray or neutralizing electron beam, needed for signal acquisition, partially reduces the Cu-ions in situ. Contrary to some prior reports such reduction by dehydration during heating up to 500°C was not observed. Heat treatment under oxidizing conditions induces changes in the populations of Cu ions in different spatial coordinations. At the same time, the severe oxy-reduction treatments do not appear to cause any Cu migration out of the zeolite framework. 2+

Copper-exchanged ZSM-5 zeolite is currently the subject of intense study because of its activity in NO decomposition and selective reduction in excess oxygen (i). While the recent literature indicates that there may be more active or selective catalysts for these reactions (2), Cu-ZSM-5 will likely be used in any case as the template for unraveling the mechanism, and to gain a deeper understanding of the operation of zeolite catalysts in general. An important part in deciphering the mechanism is the identification of the active site, which in the case of the reduction or decomposition of NO, must allow for the formation of dinitrogen; i.e. the pairing of nitrogens in NO. In the original scheme of Iwamoto (7) and in other schemes (5) this site was assigned to the cuprous ion. Considering the very strong oxidizing conditions of the reaction, the plausibility of this assignment has been called into question (4). To be able to identify different oxidation states, preferably under conditions similar to those of actual use, unambiguous experimental measurements are required.

0097-6156/94/0552-0066$08.00/0 © 1994 American Chemical Society In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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HAACK & SHELEF

X-ray Photoelectron Spectroscopy ofCu-ZSM-5 Zeolite 67

To follow the changes in the oxidation state of copper in Cu-ZSM-5 electron paramagnetic resonance (EPR) (3,5,6) is the method of choice. There are several difficulties with its use under any conditions apart from vacuum at low temperatures because even the adsorption of inert gases alters the signal from the paramagnetic C u ions located at specific sites in the zeolite (6). Here we attempt to explore whether X-ray photoelectron spectroscopy (XPS), a technique thought to be less sensitive to interferences in the identification of oxidation states, can be of use. XPS has been used previously to investigate X and Y-type Cu-exchanged zeolites (7-10) which have a relatively high A l content and concomitant high Cu-loadings, as well as to determine copper speciation in a copper-containing intercalation phase (11) and to identify the oxidation state of copper in spent catalysts on different supports (12). Recently there was one XPS study of the Cu-ZSM-5 zeolite (13). In the present work the oxidation state of copper in copper-exchanged ZSM-5 zeolite was determined following various in situ oxidizing and reducing treatments at atmospheric pressure, and the results were compared with those of a previous study (13) where heat treatments were conducted in vacuo.


2 +

Experimental Explicit details describing the XPS analytical system and attached catalytic reactor have been reported in an earlier paper (14). A summary of the experimental conditions pertaining to this study is given below. Copper-exchanged ZSM-5. The H-ZSM-5 zeolite, exhibiting a S i 0 / A l 0 molecular ratio of 30, was purchased from the PQ Corporation, Zeolites & Catalysts Division. The zeolite was 40 at.% exchanged with copper (1.31 wt.% or 0.41 at.% overall Cu, as determined by ICP-MS). The sample is close in composition to the one used in ref. 13 where the Si/Al atomic ratio was 13.4and the exchange was 46 at. %. The zeolite powder was pressed into a 6-mm diameter pellet to allow for reactor treatment and analysis by XPS. 2

2

3

XPS Analytical System. Spectra were acquired on an M-Probe XPS spectrometer manufactured by Surface Science Instruments, VG Fisons, using monochromatic AlKa X-rays (1486.7 eV, 80W) focused to a 1200-/*m diameter beam. Unless stated otherwise, total time for acquisition of the Cu core level spectrum was 10 min. The relatively low X-ray power, large spot diameter, and short acquisition time were chosen to insure that minimal reduction of the cupric ion by the X-ray flux occurred during analysis, which has been observed by others (9,10,13) for the sensitive zeolite structure. A low energy (1-3 eV) electron flood gun and a Ni charge neutralization screen placed 1-2 mm above the sample were employed to minimize surface charging effects (15). The analyzer was operated at a 150-eV pass energy. Binding energies reported for the Cu 2p core level spectra were referenced to the Si 2p line at 102.9 eV, i.e. adventitious aliphatic C Is line at 284.6 eV. All binding energy positions quoted in this work were measured to an accuracy of ± 0.2 eV. The raw data from the Cu 2p core level spectra were smoothed using the Savitsky-Golay method. Photoionization yields of the Cu 3/2

3/2

In Environmental Catalysis; Armor, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

68

ENVIRONMENTAL CATALYSIS

2p and Si 2p lines were normalized to give Cu/Si atomic ratios by means of routines based on Scofield's photoionization cross-section values (76). The samples were treated in a PHI Model 04-800 Reactor System mounted directly onto the preparation chamber of the spectrometer (14). The treatments were carried out at 500°C and atmospheric pressure for 6 h, after which the samples were cooled in the flowing reactor gases to below 100°C (about 30 min). At this point gas flow was ceased, and the reactor door was opened to vacuum. Samples were transferred between the reactor and spectrometer in vacuo to eliminate contamination and reoxidation by air. The reactor gases used were Ar (99.9995%), 0 (99.98%), H (99.9995%) and CO (99.99%), purchased from Matheson. 3/2


2

2

Results and Discussion Cu-exchanged ZSM-5 sample A was subjected to the sequence of treatments given in Table I. Each reactor treatment, at 500°C for 6 h, was followed immediately by XPS analysis. After analysis, the sample was transferred back into the reactor for the next successive treatment, and so forth. A second sample, B, was subjected to 0 and CO treatments only, to discern how CO treatment directly after oxidation compares to CO treatment after the cyclic oxidative and reductive treatments subjected to sample A. Table I also includes 9 measurements of the Cu/Si atomic ratio attained after the different treatments, plus the mean and standard deviation of these measurements. The mean Cu/Si atomic ratio of 1.58 Χ 10" was close (within a standard deviation of ± 0.27) to 1.33 Χ 10" , the value derived for the sample bulk from ICP-MS. The lowest Cu/Si ratios measured by XPS for sample A, 1.34 X 10' , 1.22 Χ 10" and 1.31 x 10" , appeared after reductive Treatment Nos. 2, 5 and 7, respectively. (Treatment No. 2, a 400-min XPS acquisition, was in essence equivalent to a reductive treatment, as will be explained later under the "Discussion" section.) However this trend was not observed in sample B, since the Cu/Si ratio measured after reductive CO treatment was 1.70, consistent with the higher ratios measured for sample A, after oxidative treatments. Nonetheless, it should be noted that even after the harsh reduction in Treatment No. 5 no substantial surface migration of Cu out of the zeolite structure was evidenced by XPS, which, assuming surface Cu at these low levels remains mostly dispersed, would have been observed as an enhanced Cu/Si atomic ratio. Figure 1 shows the Cu 2p core level spectra acquired for sample A initially, and after reactor treatments at 500°C in 0 , Ar, and again in 0 immediately following reduction in H . The sequence of these spectral acquisitions corresponds to Treatment Nos. 1,3, 4 and 6, respectively, in Table I. For all spectra, the Cu core level consisted of two main peaks at 933.5 and 936.5 eV, referred to as peaks I and II, respectively, and a corresponding broad satellite peak centered at 944.5 eV (943.5 eV in the initial spectrum). The presence of C u is confirmed by the appearance of the satellite peak (7), which is absent in Cu° and C u states. The binding energy for peak I, 933.5 eV, is similar to that observed for CuO (17). However, the binding energy noticed for peak II is considerably higher, indicating that 1) the copper oxidation state is 2

2

2

2

2

2

3/2

2

2

2 +

1 +


2

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X-ray Photoelectron Spectroscopy of Cu-ZSM-5 Zeolite 69

Table I. XPS Cu/Si atomic ratio of Cu-exchanged ZSM-5 as a function of treatment Sample

Treatment No. (Description)

Cu/Si Ratio x 10* a

1.33 1.42

1 (initial, 10-min XPS acquisition) 2 (initial, 400-min XPS acquisition) 1.34 3 (500°C 0 ) 4 (500°C Ar) 5 (500°C H ) 6 (500°C 0 ) 7 (500°C CO)

1.73 1.68 1.22 2.04 1.31

2


2

2

1.79 1.70 158 (027)

1 (500°C 0 ) 2 (500°C CO) 9

b

l

In the bulk of the Cu-ZSM-5 sample, as determined by ICP-MS. 'Mean (standard deviation), both samples. 936.5

950

945

940 935 Binding Energy (eV)

930

925

Figure 1. XPS Cu 2p core level spectra of Cu-exchanged ZSM-5 zeolite A) initially, B) after oxidation at 500°C for 6 h, C) after heating in Ar at 500°C for 6 h and D) after reduction in H at 500°C followed by oxidation at 500°C for 6 h. 3/2

2


70

ENVIRONMENTAL CATALYSIS

probably +2, and 2) that this state is highly ionic in nature; i.e. a binding energy of ~936eV is observed for CuF (18). Within the zeolite framework, this species may be more properly viewed as being coordinatively unsaturated. While the binding energy positions of both peaks I and II are consistent with divalent copper, the satellite envelope represents a combination of peaks, each corresponding to one of the C u core lines (9). However, since the peak definition under the satellite envelope is quite subtle, no attempt is made to deconvolute this region. The satellite peak is uniquely associated with the C u state because in this case two channels for core level screening, a local and nonlocal one, are possible during the photoemission process. For C u the photoelectron emitted is screened either by the 3d electrons, or by the 3d electrons plus one electron from the ligand. The former gives rise to the satellite peak, while the later, being equivalent to a full 3d configuration as is present in Cu° and Cu , contributes to the main core line. The core level peaks I and II, previously observed in Cu-ZSM-5 (13) as well as in copper-exchanged X- and Y-type sodium zeolites (9), have been assigned by Contarini and Kevan to tetrahedrally and octahedrally coordinated C u , respectively. This assignment of both peaks to the cupric ion was supported by ESR measurements where both states were shown to be associated with the paramagnetic response. It was suggested that the octahedrally coordinated C u in the zeolite was hydrated, with the cupric ions coordinated by three framework oxygens and three water molecules. In an XPS study of Cu-ZSM-5 and Cu-Y zeolite by Jirka and Bosacek (75), two peaks with similar binding energies were present. Peak II has been observed to disappear after prolonged in vacuo heat treatment and then reappear after exposure to ambient air, i.e. water vapor. For the Cu-exchanged ZSM-5 in this work, the initial Cu 2p spectrum (Figure 1A) showed a more prominent peak at 933.5 eV (peak I). After heating in 0 at 500°C the peak at 936.5 eV (peak II) was enhanced (Figure IB), while an additional heating in Ar (Figure 1C), essentially an inert heat treatment, did not change this spectrum. In addition, after a subsequent reduction in H and reoxidation in 0 (Figure ID), peak II virtually displaced peak I. Thus, treatment under oxidizing conditions gradually shifts the coordination of the cupric ions towards a configuration associated with the peak having the higher binding energy. Once the copper has been reduced to the metallic state (vide infra) and reoxidized, most copper ions, at the examined exchange level, end up in the higher binding energy state. These results differ somewhat from those of Jirka and Bosacek. However, reactor treatments in this study were done at atmospheric pressure, whereas in both refs. 9 and 13 the heat treating was accomplished in vacuo. It is postulated, based on the extensive work of Kucherov et al. (5,6) and at variance with Jirka and Bosacek, that the coordination of the cupric ions in Cu-ZSM-5 and Cu-Y zeolite differs. According to Kucherov a CuH-ZSM-5 sample exchanged to 20% by copper and dehydroxylated at 500°C, i.e. very close to that used in this work, contains two differently coordinated isolated Cu ions. Both kinds of C u ions are coordinatively unsaturated; one in a square-planar configuration and the other in a square pyramidal configuration. These unsaturated cupric ions do coordinate a wide variety of ligands upon adsorption, oxygen molecules included 2

2 +

2 +

2 +


9

9

10

1+

2 +

2 +

3/2

2

2

2

2 +


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X-ray Photoelectron Spectroscopy ofCu-ZSM-5 Zeolite

(5), so it is probable that not only hydration may induce changes in coordination leading to the observed changes in binding energy. Also, consider that coordination changes can be induced in Cu-ZSM-5 by mere adsorption of nonpolar adsorbents (6). Figure 2 compares the Cu 2p spectrum obtained initially to those obtained after reductive treatments. Spectra A, B, C and D correspond to Treatment Nos. 1, 2, 5 and 7 in Table I, respectively. In contrast to the initial spectrum of the copper-exchanged zeolite observed after a 10-min spectral acquisition (Figure 2A), after a 400-min spectral acquisition (Figure 2B) the satellite peak and peak II (936.5 eV) of the Cu 2p core line disappeared, while the peak at 933.5 eV persisted. The disappearance of the satellite peak implies a reduction of the Cu below the +2 state. The remaining peak at 933.5 eV did not shift, but the binding energy is also consistent with what has been observed previously for Cu by Jirka and Bosacek after X-ray induced reduction in Cu-ZSM-5. The Cu 2p spectrum obtained after H reduction is shown in Figure 2c. The satellite peak was absent, and the core level peak was shifted to 932.0 eV. One may conclude that after these severe reduction conditions the copper was reduced to metallic Cu°. The binding energy observed is consistent with that reported previously for Cu metal (9). Reduction by CO (Figure 2d) also caused the satellite peak to disappear, implying a reduction of the copper below the +2 state. However, a broad core level peak appeared, centered between 933.5 and 932.0 eV. It is reasonable to assume that the less severe reduction in CO may have reduced the copper to a mixture of C u and Cu° states. This agrees completely with the previous observation that the cupric ions in Cu-ZSM-5 are more easily reduced by H than by CO (5). A second reduction with CO was performed on sample B, after the sample was subjected to an initial oxidation only (Table I). The Cu 2p core level spectrum obtained from that sample was identical to that of sample A shown in Figure 2d. In principle, a more precise definition of the chemical state of copper could have been obtained by measurement of the Auger parameter (19). Such information would have been helpful to differentiate the reduced states of copper found in Figure 2. As it happens, the most intense copper Auger line, Cu L M M , is considerably broader, and only about half as intense as the Cu 2p core line. This means that at the low levels of copper present in the ZSM5 zeolite, a large acquisition time would have been necessary to obtain an accurate measurement of the kinetic energy of the Cu L M M line. During this time, any further X-ray induced reduction of copper would have been inevitable, and the measurement would no longer reflect the original state of copper associated with the Cu 2p core level obtained after a short acquisition time of 10 min. Thus, for this system, measurement of the Cu Auger parameter is impractical. 3/2


3/2

1

3/2

+

2

1 +

2

3/2

3

4 5

4 5

3/2

3

4 5

4 5

3/9

Conclusions The results of this study indicate that Cu-ZSM-5 is stable with respect to the migration of the Cu-ions out of the zeolitic framework up to 500°C under severe non-hydrothermal oxy-reductive treatments. Although explicit assignments of


71

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ENVIRONMENTAL CATALYSIS 932.0


933.5

950

945

940 935 Binding Energy (eV)

930

925

Figure 2. XPS Cu 2p core level spectra of Cu-exchanged ZSM-5 zeolite A) after a 10-min acquisition, B) after a 400-min acquisition, C) after reduction in H at 500°C for 6 h and D) after reduction in CO at 500°C for 6h. 3/2

2

configurations of Cu in the zeolite structure to the specifically observed Cu 2p lines could not be made, it was nevertheless shown that heat treatment under oxidizing conditions induces changes in the populations of C u ions in different spatial coordinations. One state is similar in binding energy to CuO, while the other state appears to be highly ionic in nature, i.e. coordinatively unsaturated. The Auger parameters necessary for the unambiguous assignment of the reduced copper states can only be obtained in the low-Cu specimens (i.e. Cu exchanged into high Si/Al ratio zeolites) when simultaneous in situ instrumentally induced reduction is made possible during the long acquisition times. This will be still more difficult to avoid in the interpretation of data obtained under less definable conditions such as those encountered in real catalytic practice. 3/2

2 +

Acknowledgment We thank George Graham for going over the manuscript and his incisive comments.


6. HAACK & SHELEF X-ray Photoelectron Spectroscopy of Cu-ZSM-5 Zeolite 73


Literature Cited 1. Iwamoto, M . ; Hamada, H. Catal. Today. 1991, 10, 57-71. 2. Misono, M . ; Kondo, K. Chem. Lett. 1991, 6, 1001-2. 3. Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T; Anpo, M . J. Catal. 1992, 136, 510-20. 4. Shelef, M . Catal. Lett. 1992, 15, 305-10. 5. Kucherov, Α. V.; Slinkin, Α. Α.; Kondrat'ev, D. Α.; Bondarenko, T. N . ; Rubinshtein, A. M ; Minachev, Kh. M . Zeolites. 1985, 5, 320-4. 6. Kucherov, Α. V.; Slinkin, A. A. J. Phys. Chem. 1989, 93, 864-7. 7. Jirka, I.; Wichterlova, B.; Maryska, M . Stud. Surf. Sci. Catal. 1991, 69, 269-76. 8. Narayana, M . ; Contarini, S.; Kevan, L. J. Catal. 1985, 94, 370-5. 9. Contarini, S.; Kevan, L. J. Phys. Chem. 1986, 90, 1630-2. 10. Kaushik, V. K.; Ravindranathan, M . Zeolites. 1992, 12, 415. 11. Guerrero-Ruiz, Α.; Rodriguez-Ramos, I.; Siri, G. J.; Fierro, J. L. G. Surf. Interface Anal. 1992, 19, 548-52. 12. Malitesta, C.;Sabbatini, L.; Torsi, L.;Zambonin, P. G.; Ballivet-Tkatchenko, D.; Galy, J., Parize, J. L.; Savariault, J. M . Surf. InterfaceAnal.1992, 19, 51318. 13. Jirka, I.; Bosacek, V. Zeolites. 1991, 11, 77-80. 14. Shelef, M . ; Haack, L. P.; Soltis, R. E.; deVries, J. E.; Logothetis, E. M . J. Catal. 1992, 137, 114-26. 15. Bryson III, C. E. Surf. Sci. 1987, 189-190, 50-8. 16. Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129-37. 17. Wolberg, Α.; Ogilvie, J. L.; Roth, J. F. J. Catal. 1970, 19, 86-9. 18. Wagner, C. D.; Riggs, W. M . ; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy, Muilenbery, G. E.,Ed.; 1st Edition; PerkinElmer Corporation, Physical Electronics Division: Eden Prairie, Minnesota, 1979, p 82. 19. Wagner, C. D.; Gale, L. H.; Raymond, R. H. Anal. Chem. 1979, 51, 466-82. RECEIVED

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X-ray Photoelectron Spectroscopy of Cu-ZSM-5 Zeolite - American

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