Esca Study of Incorporation of Copper into Y Zeolite

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Cafalysis 0 1991 Elsevier Science PublishersB.V., Amsterdam

269

ESCA STUDY OF INCORPORATION OF COPPER INTO Y ZEOLITE Ivan Jirkae Blanka Wichterlovaa and Martin Maryskab

aJ. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 182 23 Prague 8 , Czechoslovakia ’Institute of Chemical Technology, Department of Silicates, 166 28 Prague 6, Czechos1ovakia Abstract Both low and high temperature mode of contact interaction between Cu2O and NH4-Y zeolite in a mechanical mixture has been observed by means of XF'S and XAES spectroscopies. Moreover, hydration of this mixture significantly increases extent of this interaction. 1. INTRODUCTION

It has been shown that solid-state (or contact) interaction can occur among various metal compounds and zeolites in mechanical mixtures, resulting in deaggregation of the metal compound phase and migration of metal ions into the zeolite channels [l-61. Generally, both the low temperature and high temperature modes of solid-state interaction can take place 131. The detail mechanism of this interaction is not known being affected by the type of a zeolite and a metal compound in the mixture /4-61. It has been found previously that a solid-state ion exchange occurs in the mixture of Cu oxides and NHs-Y or H-ZSM-5 zeolite after heating above 670 K [61. This information belongs to the changes in the bulk of zeolite crystals. I t seems probable that some changes in the Cu20/NH4-Y (CuzO/H-ZSM-5) interface may occur at much lower temperature. The electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM) have been used to investigate the changes in the Cu20 - zeolite interface resulting from heating and hydration of the mixture. 2. EXPERIMENTAL

The mixture was prepared by mechanical grinding of Cu20 (Merck) and NH4-Y zeolite in an agate mortar for 60 minutes. The chemical composition of NH4-Y was (wt.%): SiO2 = 67.72, A1203 = 22.21, Na2O = 1.41 and (NH4)20 = 8.65. The concentration of CuzO in the mixture was 112 mg/g of zeolite, corresponding to a Cu/OH (bridging) molar ratio equal to 0.5. In some cases the mixture was exposed to water vapour (p(H20)- 440 torr) in a static air atmosphere at 358 K for 0.5, 11.0 and 20.5 hours. The spectra of the mixtures were measured without any heat treatment and after vacuum heat treatment at 420, 620 and 770 K for 1 hour. The photoelectron and Auger lines were measured on an ESCA 3 Mk I1

270

spectrometer at ambient temperature and at a base pressure typically lower than lo-* torr. The AlKa ( E = 1486.7 eV) and MgKa (E = 1253.4 eV) lines were used to excite the photoelectrons. The Cls line (Eb = 284.4 eV1 was employed to calibrate the energies of spectra. The error of Eb (Ek) estimation was typically 0 . 3 eV. An analytical information from the electron spectra may be obtained from their intensities, binding energies of the photoelectrons and kinetic energies of the Auger electrons of a given atom. As investigated mixtures were substantially heterogenous (see bellow), the Cu concentrations estimated by ESCA were only semiquantitative. The simplest equation was used: Cu/Si = I(CuZp)~(SiZp)/I(SiZp)u(Cu2p)

(1)

where Cu/Si is a copper-silicon atomic ratio, I(Cu2p) and I(Si2p) are intensities of Cu 2p3/2 and Si 2p photoelectron lines, respectively, and cr(Cu2p) and u(Si2p) are photoionization cross-sections of Cu2pw2 and Si2p levels, respectively [71. Scanning electron microscopy (SEMI was done on a JEOL JEM 1008. Accelerated voltage was 40 kV. The surface charge of the sample was compensated by evaporated layer of Pd/Au alloy. 3. RESULTS AND DISCUSSION 3.1. ESCA of Cu ions

The estimation of the location of Cu ions in the mixture is based on a fingerprint method, i.e. on comparison of copper core level binding energies Eb and kinetic energies Ek of Auger electrons of copper with standard values of Eb and Ek of Cu compounds. This method may be complicated by charging effects resulting from the emission of electrons from insulating materials like zeolites. The results then should be carefully checked, whether they are in accordance with recent interpretations of Eb and Ek values. It is known that Eb of Cu 2p3/2 line of Cul’compounds do not substantially differ each other while their kinetic energies Ek of an Auger Cu CVV transition depend on the type of Cu compound. Both the core level E b and Auger Ek values of Cu2+are dependent on the type of a compound. These effects can be explained by screening theory proposed in literature IS]. Two channels are available for the screening - local and non-local one. Two lines are then observable in the core level photoelectron spectrum of divalent copper. Lower energy main line, screened by 3d9 electrons of copper and by another electron from a ligand localized during screening in the Cu 3d orbital and a higher energy satellite screened by Cu 3d9 electrons only. According to the interpretation of van der Laan et al. [Sl the main line is for the case of divalent copper sensitive on its chemical surroundings due to screening mechanism, while the energy of a satellite is almost independent on the chemical surroundings. F o r monovalent copper only 10 one screening channel is available (3d configuration which exclude any charge transfer from the ligands) and so this line should be, according to the above interpretation, insensitive to the chemical surroundings of the Cul+ ion. The use of the Eb values of Cu 2p3/2 line of copper enable to distinguish changes in the coordination of Cu2+ ion in the investigated mixtures. Moreover, knowing the origin of the satellite in the Cu 2p3/2

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TABLE 1 The Cu 2p3/2 binding energies Eb (eV) and Cu L3M4,5M4,5 Auger kinetic energies Ek (eV) of Cu compounds and Cu ions in zeolites. Compound

Ref.

Eb

Ek

cu20 Cu20 (dispersed) cuc1 cul+-y

932.2 932.7 932.6 932.4

916. a 915.9 915.0 913.2

this work

CUO CUCl2 Cu (OH12 Cu"-Y

933.5 934.4 935.1 936.2

917.9 915.5 913.1

this work

9 10 11 8 10 11

TABLE 2 The Cu/Si. 10’ratio of CuzO/NH4-Y ( A ) unhydrated, hydrated for (B) 0.5 h., (C) 11 h., (D) 2 0 . 5 h., and heated at temperature T (K) in situ.

A

B

(4.5)

(7.5)

-

3.7 3.8 3.3

-

4.6 4.1 3. 6

C 12.2 10.7 10.6 7.1

D

T

(28.7) 23.8 19. a

293 293 420 620 770

-

2.3

number in brackets - Cu/Si ratio after 5 . 5 minutes of measurement (see the text) TABLE 3 Binding energies Eb(eV) of the Cu 2p3/2 lines of copper in hydrated Cu20/NH4-Y mixture and their atomic ratios of Cul /Si and Cu2+/Si estimated from eq. ( 1 ) af-ter 5 . 5 minutes of measurement- see the text). Eb(CU1+) 931.4 932.2 931.8

Eb(CU")

Cul+/Si.10'

934.5 934.9 935.0 935.1

2.4 2.0 3. a

-

Cu2+/Si.lo2 Hydration (h) 2.1 5.5 9.4 28.7

0.5 11.0 20.5

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spectrum (no satellite is observable in this spectrum for CulC), we can estimate the oxidation state of Cu in the mixture. Similar screening effects influence two hole Auger final states. It has been shown that kinetic energy Ek of L3M4,5M4 5 transition of both Cu2+ (3d9 and 3d8 Auger final states) and Cu1+(3d8’ Auger final state) depends on chemical surroundings of the copper ion. Our previous data on the ion exchanged Cu-Y and Cu-ZSM-5 zeolites are in accordance with the above theory. The E b of Cu 2p3/2 line of Cul+ ion in zeolites do not substantially differ from the values which belong to other cuprous compounds. On the contrary, the Ek values of the Auger Cu CVV transition of Cul+ ions in zeolites are much lower in comparison with any other Auger Cu CVV kinetic energy (see Table 1). In accordance with the screening theory the Eb of Cu 2$1*3/2line of Cu2+ in Y zeolite is much higher than that of the other Cu compounds. It follows that the Eh of Cu 2p3/2 line of Cu2+ species and the E k of Cu L3M4,5M4,5 (Cu CVV) line of both Cul+ and Cu2’ species may be used to distinguish qualitatively the coordination and, therefore, location of Cu in the mixture. 3.2. CU O/NH -Y 2 4

The SEM reveals that the unhydrated mixture was composed from the grains with a diameter of about 1 pm and from the agglomerates with a diameter of about 3 - 6 pm. The hydration of the mixture at 358 K resulted in a disappearance of these agglomerates. The structureless spots with a diameter of 10 pm appeared in the mixture hydrated for 20.5 hours. As no changes induced by hydration were observed for pure Cu20, the disappearance of agglomerates and the presence of structureless spots in the heavily hydrated mixtures may be explained by the deaggregation of CuaO caused by the zeolite induced hydrolysis. This was also indicated by the dependence of the Cu/Si intensity ratios on the time of hydration estimated by ESCA (Table 2 ) . A pronounced increase of this ratio with the time of hydration confirms deaggregation of copper oxide phase Further details on the copper oxide-zeolite interaction were gained from the binding energy values and shapes of the Cu 2p3/2 photoelectron spectra (Figure 1). Two lines abbreviated as line I and I1 (at -932 and -935 eV, respectively) with a satellite at a higher binding energy were resolved by a fitting procedure for unhydrated mixture and for that hydrated for 0.5 and 11.0 hours. The high energy Cu 2p3/2 line and a satellite disappeared during the measurement (after -240 minutes 1. The accumulation time of the Cu 2p3/2 spectra was thus minimized (5.5 minutes). The only one Cu 2.~312 line with a satellite was observed for the mixture hydrated for 20.5 hours at a binding energy Eh = 935.1 eV (not shown in Figure 1). Line I belongs to cuprous species and its Eh was slightly lowered in comparison with standard Eb values. However, this deviation (except of unhydrated mixture) was about what is expected from experimental error. A lowering of Eb of line I which belongs to unhydrated mixture was most probably caused by wrong calibration (see discussion of this problem in [ 1 2 1 ) . Alternative explanation of this effect as a consequence of charge donation from the zeolite to Cul+ species is in disagreement with the results of discussion presented below. The high energy Cu 2p3/a line with a satellite corresponds to the cupric ions bonded most probably in Cu(0H)a and no Cu2+ ions were observed by ESCA to be exchanged into the zeolite by hydration. This follows from a comparison of the binding energy Eb value

-

273 (Table 3 ) with that of the standard compounds (see Table 1). The longer time of hydration, the higher was observed concentration of Cu(0H)z. This effect seemed to be quantitative for heavily hydrated mixture, as no 1+ substantial concentration of Cu was found. Migration of some copper species into the zeolite channels occurred under vacuum during spectra measurement. This follows from a decrease in the Cu/Si ratio with time of measurement (see Table 2 and Figure 2 ) . A further decrease in the Cu/Si ratio was caused by heating of the samples at 770 K. Again, this effect was most pronounced for the heavily hydrated mixture (Table 2 ) . It follows from the above discussion that the Cu/Si ratio estimated by XPS was influenced by two effects - deaggregation of the copper oxide phase, which increases the Cu/Si value, and subsequent migration of copper species into the volume of the zeolite crystals, decreasing, on the contrary, the Cu/Si ratio. The more hydrated the copper oxide phase (corresponding to a higher initial concentration of Cu(0H)z in the mixture), the greater extent of incorporation of copper species inside the zeolite was found. The conclusion on the deaggregation and diffusion of (at least part of) the copper species into the zeolite channels is supported by discussion of the shape and energy of Auger CVV spectrum of copper. The numerical values of Ek presented here are only rough estimations being discussed here only qualitatively. Their exact values can be obtained, in principle, by curve fitting of Auger spectra. However, this procedure cannot be unfortunately used because of a lack of information on the line shapes of the fitted components, which may be very different

A

91 9

A

934

B

Eb( e v ) 949

FIGURE 1 ( A ) Cu 2p lines of copper in CuzO/NH4-Y mixture (a) unhydrated; hydrated for (b) 0.5 h, (c) 11.0 h ( B ) Typical fit of the Cu 2p line

274

B A

lop

0

0

0

0

0

0 0

0

IL . 0

I

100

., % I 200

t (min )

0

0

100

200

FIGURE 2 ( A ) Dependence of Cul+/Si (open points) and Cu2+/Si (full points) of the and hydrated ( 0 1 for 11.0 h on the time t (min) sample unhydrated ( of measurement. ( B ) Dependence of Cu/Si ratios (Cu/Si = Cu’+/Si + Cu2+/Si) of the unhydrated mixture ( c) and the sample hydrated for 11.0 h ( 0 ) and 20.5 h ( A on the time t (min) o f measurement.

a)

Figure 3 depicts the Cu CVV Auger lines of copper in the unhydrated mixture and in the samples hydrated for 11.0 and 20.5 hours. For the unhydrated mixture and f o r that hydrated for 0.5 (not shown in Figure 2) and 11.0 hours the Cu CVV spectra were composed of two lines at a kinetic 917 eV (close to the value observed for Cu20, 917.4eV) and at energy Ek Ek 913 eV (close to the value of Cul’ion exchanged in the Y zeolite, 913.2 eV). The interpretation of an additional line (at lower kinetic energy) found in this spectrum, also observed in a pure zeolite, is not yet clear. The intensity of the Auger line at 913 eV increases with the time of hydration and decreases with the heating of the mixture (Figure 2). Only one broadened Cu CVV line was observed for heavily hydrated sample 917 eV. Heating of this sample at 770 K caused a substantial line at Ek shape change and shift to 913.5 eV. These effects can be explained in terms of migration of at least part of the Cu species into the zeolite channels, even at ambient temperature (see discussion of the Cu/Si ratio above). More extensive incorporation of Cu species into the zeolite channels due to heating of the sample increases with the time of hydration, In the sample hydrated for 20.5 hours followed by heating at 770 K, all the copper species can be assumed to be incorporated into the zeolite channels (no Cu CVV line of Cul+in Cu20 was observed - see Figure 3).

-

-

-

275

I

I

I

I

1

1

915.8 FIGURE 3

I 1

912.9

Ek(eV)

916.8 912.8

Ek(eV)

I

Auger Cu CVV spectra of copper in Cu20/NII4-Y mixture ( 1 ) unhydrated; ( 2 ) and (3) unhydrated followed by heating at 620 and 770 K, resp.; (4) hydrated for 11.0 h; (51 hydrated for 11.0 h followed by heating at 770 K; ( 6 ) hydrated for 20.5 h; (7) and ( 8 ) hydrated for 20.5 h followed by heating at 420 and 770 K, resp. 4. CONCLUSIONS

The core level and Auger shifts of copper ion exchanged in zeolites compared to those in various Cu compounds may be explained by the screening theory proposed in literature. The core level Eb of Cul’ions depends only weakly on their chemical surroundings. The kinetic energy of the Auger Cu CVV line of the Cul’ion exchanged in zeolite may be used as a fingerprint^ value due to nonlocal screening of the Auger final state of the Cul+ ion. In the case of Cu2+both the core level E b and Auger E k values may be used due to the nonlocal screening of final states. The first step of contact interaction of CuzO with NH4-Y zeolite in their mechanical mixture is deaggregation of Cu20 and its oxidation to Cu(0H)z. A part of Cu species is incorporated into the zeolite channels during measurement of photoelectron spectra likely as a consequence of sample heating during measurement and/or by photodissociation of the Cu(0H)a thin layer. Heating of the mixture already at 420 K causes further migration of Cu species into the zeolit,e channels. This migration was previously indicated for the zeolite bulk after heating of the mixture

216

above 620 K. Oxidation of CuzO and a low as well as high temperature migration of Cu species into the zeolite are substantially increased by pre-exposure of the mixture to water vapour. This effect is explainable by an increased fraction of Cu bonded in Cu(0H)a due to hydration. As a lattice energy of cupric hydroxide is lower than that of cuprous (cupric) oxide, dissociation of a former compound is energetically more favourable and expectably an easier incorporation of Cu into the zeolite channels takes place.

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5.

D.W. Breck in Zeolite Molecular Sieves - Structure, Chemistry and Use, Wiley, New York, 1974, pp 588 - 592. A . V . Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38. H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79. B. Wichterlova, S. Beran, L. Kubelkova, J. Novakova, A. Smieskova and R. Sebik, Stud. Surf. Sci. Catal. 46 (1989) 347. S. Beran, B. Wichterlova and H.G. Karge, J. Chem. SOC. Faraday Trans. 86 (1990) 3033.

6. B. Wichterlova and H.G. Karge, submitted for publication. 7. J.H. Scofield, J. Electron Spectroscopy, 8 (1976) 129. 8. G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev.B 23 (1981) 4369.

I. Jirka, Thesis, J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Prague 1989. 10. Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (D. Briggs and M.P. Seah eds. 1, John Wiley, New York, 1983. 11. I. Jirka and V. Bosacek, Zeolites 11 (19911, 77. 12. T.L. Barr and M.A. Lischka, J. Am. Chem. SOC.,108 (1986) 3178. 9.