7
Zeolite Surface C o m p o s i t i o n b y X . P . S .
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J-FR. TEMPERE and D. DELAFOSSE Cinétique des Réactions Superficielles, E.R. 133, Université Pierre et Marie Curie, 75230 Parie Cedex 05, France J. P. CONTOUR Laboratoire de Physico-Chimie Instrumentale, Université Paris VII, 75221 Paris Cedex 05, France ABSTRACT In the present paper the s i l i c o n and aluminium composition of the external layers in some sodium zeolites or exchanged zeol i t e s have been investigated by X.P.S. Partial dealumination of the surface of these solids is shown to occur.
Introduction The adsorptive properties of synthetic zeolites are related to their ultramicroporous crystalline structure which confers upon them very high specific areas. But i n many cases, the part of the surface really involved in the phenomena of gas adsorptive and catalytic reactions i s only a small fraction of the total and can be limited in certains cases to the external layers of the c r y s t a l . The external surface of some synthetic zeolites has been studied by means of XtP.S. Experimental section Material. The following zeolites are used i n the study : NaA, NaX, NaY Linde molecular sieves ; NaZ, HZ Norton zeolons ; p a r t i a l l y calcium exchanged NaA, NaX, NaY and NaZ. Some experiments were performed with Ketjen silica-aluminas conlaining either 14% or 26,4% by weight of alumina. A series of these oxides, progressively dealuminated i n order to obtain samples containing from 0.1% to 26.4% by weight of alumina, was obtained from Dr. D. Barthomeuf. X.P.S. X-ray photoelectron spectra were recorded on an ΑΕΙ ES 200B spectrometer with a magnesium anode i n the X-ray source (1253,65 eV). The binding energies were calculated by 76 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
7.
TEMPERE
E T
Zeolite
AL.
Surface Composition
by XPS
77
taking the energy o f the Is e l e c t r o n o f carbon contaminaxion as an i n t e r n a l standard (JL, 2 ) . In view o f p r e v i o u s work, t h i s ener gy has been f i x e d at 285 eV. r e l a t i v e to the Fermi l e v e l ( 2 ) . The spectrometer i s equipped with an independent sample h a n d l i n g chamber, and a d i r e c t i n t r o d u c t i o n system. During r e c o r d i n g o f the spectrum the pressure i s 2 X 1 0 " t o r r , when the sample i s introduced v i a the p r e p a r a t i o n chamber, and 5X10~ t o r r when i t i s introduced v i a the d i r e c t i n t r o d u c t i o n l o c k . The z e o l i t e pow ders are compressed onto a copper g r i d and then, f i x e d on the sample holder whose temperature can be v a r i e d from - 180°C t o 450°C. When the samples are desorbed " i n s i t u " , they are i n t r o duced v i a the p r e p a r a t i o n chamber and t r e a t e d under vaccum (1CT t o r r ) at 300°C f o r 16 hours. The inhomogeneity o f the chemical composition i n the e x t e r n a l l a y e r s can be r e v e a l e d by an i n depth a n a l y s i s . In the case o f z e o l i t e , the use of a s p u t t e r i o n gun i s not s u i t a b l e when the samples are heterogenous i n s u l a t o r oxides, so that we t r y t o o b t a i n an i n depth a n a l y s i s by using a v a r i a b l e angle sample h o l der (3, 4 ) . Such a d e v i c e p r o v i d e s a non d e s t r u c t i v e a n a l y s i s over a t h i c k n e s s o f about one escape depth (5). Since the samples aire compacted powders, the surface roughness o f samples must be taken i n t o account i n a study o f the angular d i s t r i b u t i o n . The s i n u s o i d a l l y rough surface model which i s proposed by C S . Fad l e y (3) was chosen to d e s c r i b e the s u r f a c e roughness o f our com pacted z e o l i t e ( F i g . 1 ) . Scanning e l e c t r o n microscopy o f the s u r f a c e shows that the roughness parameter (a/λ) i s c l o s e t o u n i t y . F i g . 2 shows the average value o f the a c t u a l e l e c t r o n emission angle θ e v a l u a t e d over the e l e c t r o n unshaded area on a s i n u s o i d a l l y rough s u r f a c e f o r 0.5 < a/λ < 2.0. I t i s c l e a r that when the apparent emission angle θ decreases the a c t u a l angle Θ decreases more and when θ i s s m a l l , Θ becomes g r e a t . Such a θ v a r i a t i o n w i l l exaggerate the s u r f a c e species, not only f o r l a r g e values o f Θ, but a l s o f o r the smallest ones. 10
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9
8
?
1
1
τ
Results Energies o f the A l and S i e l e c t r o n s i n the z e o l i t e s . The energies o f the A l ^ and Si2p e l e c t r o n s i n d i f f e r e n t z e o l i t e s can be grouped according t o the value of the r a t i o ( A l / S i ) and the mean values are l i s t e d i n t a b l e I. The 2s l e v e l of A l does not appear tcjjbe d i s p l a c e d and the values are c l o s e t o those measured f o r γ AI2O3. On the other hand, the energies o f the SÎ2p e l e c t r o n s increase (+ 1.3 eV), on going from z e o l i t e A t o Z. The d i f f e r e n c e (Εβ s i " B A l ^ s ^ increases i n the same way. For the Y and Ζ z e o l i t e s , the b i n d i n g energy i s c l o s e r t o that measured f o r s i l i c e , f o r type A and X i t i s c o n s i d e r a b l y l e s s . s
E
2
Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
Downloaded by UNIV OF TEXAS AT AUSTIN on July 31, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch007
78
MOLECULAR
SIEVES—Π
Figure 1. Model of a general rough surface contour with both x-ray and electron shad ing (after Ref. 3)
O
10
20
30
40
50
60
70
80
90
θ° Figure 2. Curve showing the variation of θ ' with θ for various values of the roughness parameter (after Ref. 3)
Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
7.
TEMPÈRE
Zeolite Surface Composition
E T A L .
Table I .
Sample
A
B
E
s
LA1,
