ESCA study of Cu2+-Y and Cu2+-ZSM-5

ESCA study

of Cu*+-Y

and Cu*+-ZSM-5

I. Jirka and V. BosZek rovskj Institute of Physical Chemistry and Sciences, Dolejs’kova, Prague, Czechoslovakia

The Aca

The oxidation state of copper measurement is investigated. by Auger parameter analysis. temperature, is the desorption Keywords:

ESCA;

XPS;

Cu ‘+-Y;

Electrochemistry,

and Cu2+-ZSM-5 after irradiation in Cu 2’-Y The Cu2+Cu1+ reduction resulting from water The limiting step in this reaction, which of the O2 and H20 produced.

Cu*+-ZSM-5;

zeolite

Czechoslovak

by X-rays during ESCA decomposition is verified can occur at ambient

surface

INTRODUCTION Partially exchanged Cu ‘+ forms of zeolites exhibit important catalytic properties, which are discussed mainly in terms of different oxidation states and concentrations of Cu, and different modes of coordination and location of Cu’+ (Cu”) cations and CuO atoms.’ To characterize these catalysts, various types of spectroscopy have been employed. In addition to methods such as e.s.r., i.r., u.v.-VIS, X-ray diffraction, etc., electron spectroscopy for chemical analysis (ESCA) has been widely used. Estimation of the Si/Al ratio’-’ and concentration of metal ions in the zeolite framework” and their oxidation states are the most common subjects of these investigations. As the ESCA information depth is small (- 5 nm), all the information obtained by this method corresponds to the zeolite surface region. Cu’+-ZSM-5 and Cu’+ -Y zeolites have been studied by ESCA in the present work. Some changes in the copper electron spectra occurring during the measurement (i.e., X-ray irradiation) have been observed previously.“.’ Their origin is analyzed on the basis of the Auger parameters of copper.

The cupric form of ZSM-5 zeolite has the composition CU,.~~ Hs:T4 (AlO&es (Si02)89.35 per elementary cell and exhlblts a sorption capacity of 4.9 mmol/g of argon. The photoelectron and Auger spectra were measured on an ESCA III Mk 2 spectrometer (VC Scientific) using AlKa X-rays at 1486.7 eV (220 W) as the excitation source. Zeolite was put on the sample holder of the specimen introduction rod from aqueous suspension. The measurement was carried out after evacuation in the preparation chamber of: spectrometer. The base pressure during measurement was of the order of - lo-’ Torr. Spectra were measured at 77 and 293 K and after heating to 773 K for 60 min in a vacuum. The electron line intensities were estimated using a linear background. The binding energy El, calibration of the Cu 2p:slg line was carried out using the El, values of Si 2~ (102.35 eV for zeolite Y and 102.9 eV for zeolite ZSM-5), 0 1s (531.55 and 531.25 eV, respectively), and C 1s (284.4 eV) lines, as recommended in the literature.x The reproducibility of the thus-estimated El, values was typically f 0.3 eV. For estimation of the copper oxidation state, its modified Auger parameter (I was used:”

EXPERIMENTAL The cupric forms of the Y and ZSM-5 zeolites were prepared from the original zeolites by ion exchange from a CuSO., solution. After ion exchange and washing, the zeolites were dried at 100°C. Chemical analysis gave the composition of an elementary cell as Cu12.7 Na2s.6 (A102)5-1 (SiOe),s8 for the Y type. A well-developed structure was confirmed by X-ray diffraction and by sorption capacity of argon measured at 78 K (10.4 mmol of argon/g of dried zeolite). Address reprint requests to Dr. Jirka at The J. Heyrovskq Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, DolejSkova 3, 182 23, Prague Czechoslavakia. Received 18 December 1989; accepted 5 March 1990

0

1991

Butterworth-Heinemann

ar = Eh(A) + E,, (C)

(1)

E,, (C) is the copper

core level binding energy (Cu 2~y,y in the present case), and Eh (A) is the Auger kinetic energy. The El, values of electrons emitted by L3M.1,9M 1,9transition is typically used for copper, but the corresponding Auger line overlaps in investigated CuY zeolite the ‘S KLlLl Auger line of sodium. For this reason, the second most intense Cu LJMP.JM.I,~ Auger line was used for estimation of 0~.

RESULTS 8,

The sets of Cu 21:312spectra of CuZSM-5 and CuY catalysts irradiated during experiment at 293 K are

ZEOLITES,

1991, Vol 11, January

77

ESCA

study

of Cuz+-Y

and

CU’+-ZSM-5:

1. Jirka

and

V. Bos6Eek

rapidly decreases after 10 or 150 min of measurement for CuZSM-5 and CuY, respectively. The Cu2ps,z line-shape changes were much less pronounced for the sample measured at 77 K. The spectra depicted in Figzm 3 correspond to zeolite Y after prolonged irradiation and after heat treatment (773 K). The shapes and positions of the Cu 2fi3j2 spectra that correspond to the irradiated and heated samples were identical. The same result was obtained for zeolite CuZSM-5 (not shown in Figure 3). The dependence of the full-widths at halfmaximum (FWHM) of the Cu 2~3,~ lines of CuY and CuZSM-5 (measured at 293 K) on the irradiation time 1 (minutes) is depicted in Figuw 4. The FWHM of the Cu 2fls12 line increased up to - 20 min for CuY and then gradually decreased and did not change after t - 80 min. The FWHM of the Cu 2&/z line of

1

933

940

Figure 1 Cu 2p,, spectra irradiated by X-rays during min, (c) 37 min

E.b( eV)

of CuZSM-5 measured at 293 K and an experiment for (a) 3 min. (b) 6

932

938

Ebb-)

Figure 3 Cu 2p,, spectra of CuY, irradiated by X-rays during an experiment for 174 min at ambient temperature (a) and after heating to 773 K for 60 min (b) FWHM

I

(eV)

0

0

l 6 -.

O

ooo

0

0 0

0 936

942

Eb( ev)

Figure2 Cu 2p,, spectra of CuY, irradiated by X-rays during an experiment for (a) 3 min (77), (b) 117 min (77), (c) 3 min (293), (d) 114 min (293). Numbers in brackets are the temperature of the sample (in Kelvin) during the measurement

0

0 4 O0

.o 0

0 0

depicted in Figures 1 and 2. The first two spectra that correspond to CuY (Figure 2) were measured at 77 K. The accumulation time of the spectra was minimized (- 2 min) compared with the total irradiation time. The Cu 2ps12 lines were accompanied by a shake-up satellite (- 8 eV from the main line), whose intensity

78

ZEOLITES,

1991, Vol 11, January

0

0

0

II 10 Figure 4 maximum CuZSM-5;

0 0

0:. .

30

Dependence of the Cu (FWHM, eV) on time (0) CuY

50

t

2p,, line (minutes)

0

.a T (min)

full-widths at halfof irradiation: (0)

ESCA Table 1 investigated

Auger parameters (Y (eV) of Cu exchanged in the zeolites and some Cu standard values of (Y

Sample

Treatment

CUY

78 127 330 95

CuZSM-5

Cu*O cue Cu’ (metal)

min irr. min irr. min irr. min irr. (After rehydration) DT - 773 K 86 min irr. 127 min irr. 134 min irr. (After rehydration)

a 1766.4 1766.15 1766.0 1766.0 1766.5 1766.1 1766.2 1766.5 1768.7 1771.5 1770.7

CuZSM-5 decreased rapidly with time and did not change after - 15 min of irradiation, reaching the same value as in the former case. The Cu 2/1s,2 spectra of zeolite Y consist of two lines7 - a low energetic one (at - 932.5 eV), designated in the subsequent text as line I, and a highenergy one (at - 935.5 eV), designated as line II. Separation of lines I and II (- 3 eV) did not change during the experiment in both the investigated systems. The overall intensity of the Cu 2p:,,., line also did not change during the experiment. The main difference in the observed effects between investigated systems was the rate of the Cu 2~~~. line-shape changes. The intensity decrease of line II occurred in a time is comparable with the time of measurement for CuZSM-5, whereas this decrease was much slower in CuY. The binding energies El, if the Cu 2p:rlg line were 1 eV higher in CuZSM-5 than in CuY. The observed shift may be caused, among other effects, by the various contact potentials of the investigated zeolites with the spectrometer (i.e., by a reference level shift), which is difficult to estimate. This problem was discussed in great detail in the literature.s*‘n*” Through the above-mentioned difficulty, the binding energies of the Cu 2~.JPL lines are discussed only briefly here. The investigated line-shape changes were reversible. This follows from experiments in which irradiated samples were rehydrated overnight in air. The Cu 2/~,,~ line shapes of the rehydrated samples were the same as in the first series of experiements. The copper Auger parameter values for the investigated samples and some standards, mentioned in the subsequent discussion, are summarized in Table I.

DISCUSSION For CuY, the observed effects may, in agreement with the literature, be explained as resulting from a change in the coordination number of copper in the zeolite framework.G*7 According to this interpretation, line II should be assigned to a cu ric ion in octahedral coordination and line I to Cu ‘+ in tetrahedral coordination.

study

of Cu *‘-Y

and

CU*‘-ZSM-5:

1. Jirka

and

V. Bos&?k

Octahedral coordination of CL]‘)+ in the hydrated zeolite Y could occur in several ways, Cupric hexaquo complexes were proposed to be present in zeolite supercages; however, cupric ions can also be octahedrally coordinated by three framework oxygens in the SI’ position and by three water molecules.‘” The observed decrease in the intensity of line II thus reflects dehydration of the zeolite during the experiment in the above interpretation. The reversibility of the Cu 2/+4/zline-shape changes as well as their arrest at low temperature seem to be in agreement with this interpretation. The explanation given above was supported’ by e.s.r. analysis of CL?+-Y. No c1ecrease in the Cu’+ concentration was observed by e.s.r. after irradiation of the sample during the ESCA experiment. It was then pointed out that no Cu”+ self-reduction occurred in this case. The Cu 2j-)3P,line-shape changes for Cu’+-ZSM-5 can be explained analogously. The much higher rate of these changes could be explained as being a consequence of an order higher Si/AI ratio in CuZSM-5 than in CuY. The former sample is thus much less hydrated than is the latter one, and its dehydration during experiment should thus be much faster. Nevertheless, the above interpretation may be criticized as e.s.r. is not a surface-sensitive technique like ESCA, and comparison of the results obtained by these two methods may be complicated. Observed radical intensity decrease of the Cu 2~9~2 shake-up satellite in the Cup+ s ectrum is understandable as nearly quantitative Cu-8 Cu’+ (Cu”) reduction as well (see F&WW,S1 and 2). This satellite is typical for the ri” configuration of Cu2+. Its intensity depends on various factors and decreases in the absence of JahnTeller distortion’.‘” (i.e., for Cu"+ in tetrahedral coordination); nevertheless, quantification of this effect is not clear at present. No shake-up satellites has been found in the spectrum of any Cu’+ or Cu” compounds. The separation of lines I and II is close to the shift among the values of the Cu 2pyj2 lines of CUE+ and Cu’+ bonded in various compounds or between Cu”+ bonded in the same way and Cu”.‘.’ The oxidation state of the metal in a zeolite framework can, in principle, be directly estimated by Auger parameter (Yanalysis. The main advantage of the use of (Y is its independence of charging effects,” which complicate analysis of El, in insulting samples. On the other hand, Auger parameters are dependent not only on the oxidation state of the investigated atom (ion), but also on the mode of its coordination, on the potential inside the zeolitic framework, and 011 relaxation effects. Detailed interpretation of all these effects is not yet possible. Auger parameter analysis is based on the comparison of the investigated 01 values with the standard ones. As the accumulation time of the Cu L:IM~,:~M 1.5 Auger spectrum was - 90 min, Auger parameters were estimated for the samples irradiated not less than 120 min (for CuY) and 30 min (for Cu-ZSM-5), where any changes in the Cu 21:s~~line shapes were negligible.

ZEOLITES,

1991, Vol 71, January

79

ESCA

study

of CL/ ‘+-Y

and

CL?‘+-ZSM-5:

1. Jirka

and

V. BosBEek

The estimated Auger parameters of Cu are summarized in Table 1. The copper Auger parameters-of Cu20, CuO, and Cue (metal), also summarized in Table 1, were used as first standard values. The estimated values of OL of Cu in CuY and CuZSM-5, which are similar, are the closest to the value of 01for the cuprous ion in Cu20. The decrease in this value (- 2.7 eV) can be explained in the zero-order approximation as a consequence of the small polarizability of the zeolite framework compared with cuprous oxide. Further verification of the above conclusion is based on comparison of the (Y values for the irradiated samples with a for the sample heated to 773 K in situ (see Table 1). It has been shown by several methods in the literature’2*‘5*‘6 that the Cu*+ ion is reduced to Cu’+ during this treatment in zeolite Y. We suggest that similar reduction also occurs in zeolite ZSM-5. Cupric ion reduction by heating is a result of the ability of zeolites to decompose water into oxygen and hydrogen, when they are exchanged by a suitable ion.‘*’ Cupric ion reduction can be described as a three-step process: 1. H 0 ionization 2Cu %+ +2H20+2Cu2++2H1++20H1-

(2)

2. 02, Hz0 desorption 2OH’-+H20+ 1/202+2e-

(3)

3. Cu*+ reduction 2 Cu*+ + 2e-+ 2 Cu’+

14)

of the Cu’+ reduction More detailed description process including explicitly the influence of zeolite The fra_mework may be found in the literature.’ Cu’+ reduction was verified for temperatures above - 573 K by e.s.r. and i.r. spectroscopies and the course of the reaction by mass spectroscopy. This reaction is reversible13; in the air, cuprous ions are reoxidized in the following reaction: H20 + l/20* + 2 Cult

+ 2 Cu’+ OH-

(5)

As the values of 01 for the irradiated samples are very close to those for the heated ones (see Table I), it follows that Cu’+ ions were reduced to Cu’+ during the ESCA measurement. The observed effects resulting from rehydration of the samples can be described by Equation (5). The limited step in the Cu*+/Cu’+ reduction is O2 and Hz0 desorption (i.e., step 2), which can be influenced by sample surface heating during measurement. Unfortunately, as mentioned in the literature, estimation of this heating is very compli-

80

ZEOLITES,

1991, Vol 11, January

cated (if possible), quantified.

and thus this effect

cannot

be

CONCLUSION It has been shown that the Cu 2~312 line-shape changes for cupric ions exchanged in zeolite Y are caused mainly by their reduction to Cu’+ during the ESCA experiment. Similar (but much faster) effects understandable in the same way have been observed for the zeolite, CuZSM-5. The Cu*+ reduction is caused by the decomposition of water in zeolite. This decomposition occurs at a much lower temperature than 573 K, where it was observed in the literature.13 The limiting step in this reaction is desorption of the O2 and H20 formed. Electron spectroscopy for chemical ana$fsis permits measurements of the early stages of Cu reduction in the surface layers of zeolites, which is difficult to study by any other bulk-sensitive method. The mode of coordination of copper ions in the zeolite framework could influence their electron spectra as well, but an interpretation of this effect is more complicated than that proposed in literature.

REFERENCES 1 Mot-tier, W.J. and Schoonheydt, R.A. Prog. SolidState Chem. 1985,16,4 2 Andera, V., Kubelkova, L., Novakova, J., Wichterlova B. and Bednarova, S. Zeolites 1985, 5, 67 3 Gross, Th., Lohse, U., Engelhardt, G., Richter, K.H. and Patzelova, V. Zeolites 1984, 4. 25 4 Wichterlova, B., Novakova, J., Kubelkova, L. and Jiru, P. in Proceedings of the 5th International Conference on Zeolites (Ed. L.V.C. Rees) Heyden Press, London, 1980, p. 373 5 Barr, T.L. in Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (Eds. D. Briggs and M.P. Seah) John Wiley, New York, 1983, p. 318 6 Narayana, M., Contarini, S. and Kevan, L. J. Catal. 1985, 94, 370 7 Contarini, S. and Kevan L. J. Phys. Chem. 1986, SO, 1630 8 Barr, T.L. and Lischka, M.A. J. Am. Chem. Sot. 1986, 108, 3178 9 Wagner, C.D., Gale, L.H. and Raymond, R.H. Anal. Chem. 1979.51.466 10 Edgell, M.J., Baer, D.R. and Castle, J.E. Appl. Surf. Sci. 1986, 26,129 11 Lewis, R.T. and Kelly, M.A. J. Electron Spectrosc. 1980, 20, 105 12 Naccache, C. and Ben Taarit, Y. in Proceedings of the Symposium on Zeolites, Szeged, 1978, p. 23 13 Okamoto. Y.. Fukino. K.. Imanaka. T. and Teranishi, S. J. ~. Phys. Chem..lS83 87; 1983 14 Briggs, D. and Seah, M.P., Eds. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley, New York, 1983 . 1977, 81, 1527 15 Kassi, P.H. and Bishop, R.J. J. Phys. Chem. 16 Iwamoto, M., Maruyama, K., Yamazoe, N. and Seiyama, T. J. Phys. Chem. 1977, 622