Application of Surface Analysis Techniques in the Study of Catalyst

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4 Application of Surface Analysis Techniques in the Study of Catalyst Systems D. D. Hawn, R. C. Cieslinski, andH.E.Klassen Analytical Laboratories, The Dow Chemical Company, Midland, MI 48667 The application of x-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) to the solution of complex real world catalyst problems is often difficult and frustrating. Three recent developments in this laboratory have greatly aided in the solution of such problems. These are 1) a dual anode x-ray source using a silicon target, 2) an off-axis, low cost reaction facility for carrying out simple preparative treatments, and 3) installation of a dedicated high-performance Scanning Auger Microprobe (SAM) system. Advantages of silicon x-radiation include the access of aluminum and magnesium core level (1s) lines and the corresponding (KLL) Auger transitions for chemical state identification and improved quantitation, because these lines are at least 10 times more intense than the corresponding (2p) or (2s) lines. The construction of an off-axis reactor has produced a simple, versatile and inexpensive system easily adapted to any vacuum system. The role of AES and SAM in catalyst research will also be highlighted by examples. The a p p l i c a t i o n o f s u r f a c e a n a l y t i c a l t e c h n i q u e s , most n o t a b l y X-ray P h o t o e l e c t r o n S p e c t r o s c o p y (XPS) and Auger E l e c t r o n S p e c t r o s c o p y (AES), o r i t s s p a t i a l l y r e s o l v e d c o u n t e r p a r t , Scanning Auger M i c r o a n a l y s i s (SAM), i s o f g r e a t v a l u e i n u n d e r s t a n d i n g the performance of a c a t a l y s t . However, t h e r e s u l t s o b t a i n e d from any o f t h e s e t e c h n i q u e s a r e o f t e n d i f f i c u l t t o i n t e r p r e t , e s p e c i a l l y when o n l y one t e c h n i q u e i s used by i t s e l f . In t h i s a r t i c l e we d e s c r i b e n o v e l approaches aimed a t making s u r f a c e a n a l y t i c a l d a t a e a s i e r t o o b t a i n and i n t e r p r e t . These i n clude: 1) a s m a l l , l o w c o s t r e a c t i o n f a c i l i t y d e s i g n e d t o work o f f - a x i s t o a commercial XPS system, 2) a s i l i c o n anode x - r a y source f o r access t o h i g h e r b i n d i n g energy p h o t o e l e c t r o n l i n e s , and 3) t h e use o f d e d i c a t e d SAM i n c o n j u n c t i o n w i t h XPS i n c a t a l y s t problems. Examples showing the u t i l i t y o f each will be d i s c u s s e d . 0097-6156/ 85/ 0288-O037$06.00/ 0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

38


Experimental XPS s p e c t r a were o b t a i n e d u s i n g a P e r k i n - E l m e r P h y s i c a l E l e c t r o n i c s (PHI) 555 e l e c t r o n s p e c t r o m e t e r equipped w i t h a double pass c y l i n d r i c a l m i r r o r a n a l y z e r (CMA) and 04-500 d u a l anode x - r a y s o u r c e . The x - r a y source used a c o m b i n a t i o n m a g n e s i u m - s i l i c o n anode, w i t h c o l l i m a t i o n by a shotgun-type c o l l i m a t o r ( 1 ) . AES/SAM s p e c t r a and photomicrographs were o b t a i n e d w i t h a P e r k i n - E l m e r PHI 610 Scanning Auger M i c r o p r o b e , w h i c h uses a s i n g l e pass CMA w i t h c o a x i a l lanthanum hexaboride (LaBe) e l e c t r o n gun. R e a c t i o n s were c a r r i e d o u t u s i n g a f a c i l i t y mounted o f f - a x i s t o the l o a d l o c k o f t h e PHI 555 e l e c t r o n s p e c t r o m e t e r . This device u t i l i z e s a r a d i a n t h e a t e r t o h e a t a sample and mount i n s i d e a g l a s s r e a c t o r . T h i s d e v i c e i s shown i n F i g u r e 1. I t c o n s i s t s o f a c e n t e r s e c t i o n c o n s t r u c t e d from two Pyrex g l a s s - t o - m e t a l a d a p t e r s on 2.75 i n c h Confiât f l a n g e s j o i n e d t o g e t h e r . The c e n t e r g l a s s s e c t i o n i s one i n c h i n d i a m e t e r by 2.5 i n c h e s i n l e n g t h . A d o u b l e - s i d e d 2.75 i n c h Confiât f l a n g e a l l o w s f e e d t h r o u g h f o r gas i n l e t , o u t l e t and e v a c u a t i o n , and chromel/alumel thermocouple. The r e a c t o r i s sepa r a t e d from t h e sample l o a d l o c k by a h i g h vacuum gate v a l v e . A 0.125 i n c h t h i c k sample h o l d e r i s moved from t h e s t a n d a r d t r a n s p o r t / a n a l y s i s r o d o f t h e PHI system t o t h e h e a t e r zone o f t h e r e a c t o r v i a a 0.25 i n c h diameter s t a i n l e s s s t e e l r o d . A d o v e t a i l mount i s used to r e c e i v e t h e sample h o l d e r on t h e PHI probe. The 0.25 i n c h r o d i s s e a l e d by a p a i r o f g r a p h i t e - i m p r e g n a t e d T e f l o n s e a l s , r i d i n g i n a h o u s i n g c o n s t r u c t e d on a 2.75 i n c h Confiât b l a n k f l a n g e . The space between t h e s e a l s i n pumped, as i s t h e r e a c t o r v i a t h e gas o u t l e t , by a m e c h a n i c a l r o t a r y pump equipped _ w i t h l i q u i d n i t r o g e n t r a p . The e n t i r e assembly o p e r a t e s from 10 t o r r t o 1 atmosphere o f p r e s s u r e when employing t h e t u r b o m o l e c u l a r pump w h i c h pumps t h e PHI load lock. H e a t i n g o f t h e mount i s accomplished by a m o d i f i e d h a l o g e n p r o j e c t i o n b u l b , w i t h t h e sample mount temperature r e g u l a t i n g t h e a p p l i e d lamp power v i a a d i g i t a l temperature c o n t r o l l e r . A stain l e s s s t e e l i n s u l a t e d c l a m - s h e l l e n c l o s u r e houses t h e lamp, and has p r o v i s i o n s f o r lamp c o o l i n g and an i n t e g r a l r e f l e c t o r . Temperatures to 600°C i n 1 atmosphere o f hydrogen can be a c h i e v e d a t t h e sample mount. R e a c t i v e gases a r e p r e h e a t e d b e f o r e passage over t h e sample by t r a v e l l i n g t h e l e n g t h o f t h e r e a c t o r d i r e c t l y i n f r o n t o f t h e h a l o g e n lamp b e f o r e making c o n t a c t w i t h t h e sample. Alternately, gas c a n be p r e h e a t e d i n a tube f u r n a c e b e f o r e e n t r y i n t o t h e r e a c tor. However, i n t h i s case care s h o u l d be e x e r c i s e d t o n o t exceed 450°C d u r i n g a r e a c t i o n . The g l a s s - t o - m e t a l a d a p t e r s used here f o r t h e c e l l body a r e i n t e n d e d o n l y t o be used t o 400°C, w h i l e t h e g l a s s i t s e l f can s a f e l y w i t h s t a n d 500°C. Due t o t h e f o c u s s i n g power o f t h e lamp onto t h e mount, a sample temperature o f 600°C c a n be a c h i e v e d a t t h e sample w h i l e t h e g l a s s o r g l a s s - t o - m e t a l adapters remain s i g n i f i c a n t l y below t h i s temperature. F o r t h e copper/aluminum c a t a l y s t a n a l y s e s l a t e r d e s c r i b e d , t h e sample mount was t r a n s f e r e d from t h e XPS system f o l l o w i n g t r e a t m e n t and a n a l y s i s t o an i n e r t atmosphere d r y box w i t h o u t a i r exposure. From t h e r e i n d i v i d u a l p e l l e t s were t r a n s f e r r e d t o t h e SAM f o r sub sequent a n a l y s i s w i t h o u t a i r exposure. The r e v e r s e p r o c e s s was employed f o r t h e next r e a c t i o n c y c l e . 6

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


HAWN ET A L .

Surface Analysis Techniques

f F i g u r e 1. Schematic Diagram o f t h e o f f - a x i s r a d i a n t heated r e a c t o r . A. c e l l body; B. l i n e a r / r o t a r y motion f e e d t h r o u g h ; C. t r a n s p o r t r o d ; D. p r o j e c t o r b u l b ; E. r e f l e c t o r ; F. i n s u l a t e d s t a i n l e s s s t e e l e n c l o s u r e ; G. a i r c o o l i n g p o r t ; H. gas i n l e t ; I . gas o u t l e t / p u m p i n g p o r t ; J . c h r o m e l / a l u m e l thermo c o u p l e ; K. h i g h vacuum gate v a l v e ; L. sample mount.



40


Discussion 1) O f f - A x i s R e a c t i o n C e l l The a b i l i t y t o t r e a t c a t a l y s t m a t e r i a l s under c o n t r o l l e d c o n d i t i o n s f o l l o w e d by t r a n s f e r t o t h e a n a l y s i s environment w i t h o u t a i r expo s u r e , has been an i m p o r t a n t f a c t o r i n t h e s o l u t i o n o f many c a t a l y s t problems. E a r l y d e s i g n s o f such components have i n v o l v e d the d i r e c t h e a t i n g of the m a t e r i a l i n a p r e p a r a t i o n f a c i l i t y d i r e c t l y adjacent t o t h e s p e c t r o m e t e r , w i t h subsequent t r a n s f e r o f t h e m a t e r i a l be tween t h e s e two chambers. A more s o p h i s t i c a t e d m o d i f i c a t i o n o f t h i s approach, as d e s c r i b e d by Ganschow and co-workers ( 2 ) , u t i l i z e s a sample module w h i c h can be s h u t t l e d among s e v e r a l i n t e r c o n n e c t e d a n a l y t i c a l chambers and a r e a c t i o n f a c i l i t y . A l t e r n a t e d e s i g n s have i n v o l v e d t r a n s f e r o f the sample t o a remote f a c i l i t y f o r r e a c t i o n ( 3 , 4 ) , o r s e a l i n g o f t h e sample i n a r e a c t o r housed i n t h e UHV a n a l y s i s chamber ( 5 , 6 ) . A r e v i e w o f t h e f e a t u r e s o f each d e s i g n has been assembled by Ganschow and co-workers ( 2 ) . The d e s i g n d i s c u s s e d h e r e i n has t h e advantage o f b e i n g i n e x p e n s i v e and e a s i l y r e p l a c e d o r repaired. W i t h i t s o f f - a x i s d e s i g n , t h e s p e c t r o m e t e r can s t i l l be used w h i l e a r e a c t i o n i s b e i n g c a r r i e d o u t . The d e s i g n a l s o p r e s e n t s a b u f f e r between t h e r e a c t o r and a n a l y s i s chamber, thus l i m i t i n g c o n t a m i n a t i o n o f t h e a n a l y s i s chamber by t h e r e a c t o r . Adverse w a l l r e a c t i o n s a r e l i m i t e d , s i n c e the a r e a i n t h e heated zone i s g l a s s , and o n l y t h e sample and i t s h o l d e r a r e m a i n t a i n e d a t the r e a c t i o n t e m p e r a t u r e . A carbon s u p p o r t e d molybdenum c a t a l y s t used f o r the p r o d u c t i o n of L i q u i f i e d P e t r o l e i u m Gas (IPGs) demonstrates t h e u s e f u l l n e s s and h i g h temperature c a p a b i l i t i e s o f t h i s f a c i l i t y . F i g u r e 2 compares the molybdenum (3d) s p e c t r a f o l l o w i n g extended t r e a t m e n t s i n h e l i u m and hydrogen a t 500°C. F o r t h e s t a r t i n g c a t a l y s t , the Mo(3d 5/2) p h o t o l i n e i s c e n t e r e d a t 232.4 eV b i n d i n g energy when r e f e r e n c e d t o the g r a p h i t i c s u p p o r t carbon ( I s ) p h o t o l i n e a t 284.3 eV. This b i n d i n g energy i s c o n s i s t e n t w i t h t h a t r e p o r t e d f o r M o ( V I ) , o r Mo0 (4*7). F o l l o w i n g h i g h temperature exposure t o h e l i u m ( F i g u r e 2 B ) , a s e r i e s o f l o w e r b i n d i n g energy peaks were o b s e r v e d , w i t h Mo(3d 5/2) components c e n t e r e d a t 231.1 and 228.9 eV b i n d i n g energy. These a r e a t t r i b u t e d t o Mo(V) and M o ( I V ) , r e s p e c t i v e l y , based on p r e v i o u s l y r e p o r t e d l i t e r a t u r e v a l u e s ( 4 ) and work performed i n t h i s l a b o r a t o r y . I n a d d i t i o n , a h i g h e r b i n d i n g energy component a t 234.2 eV i s observed i n t h i s spectrum, and i s a t t r i b u t e d t o molybdate (Mo04=). We a l s o observed a marked i n c r e a s e i n p o t a s s i u m a t t h e s u r f a c e f o l lowing t h i s treatment. In this c a s e , t h e Mo (IV)/Mo (VI) and Mo(V)/Wo(VI) r a t i o s a r e 0.2 and 0.13 r e s p e c t i v e l y a f t e r 6 hours o f treatment. Extended r e a c t i o n t i m e s ( t o 12 h o u r s ) do not s i g n i f i cantly affect this ratio. R e d u c t i o n i n hydrogen a t 500°C produces s u b s t a n t i a l l e v e l s o f Mo, as evidenced by t h e appearance o f a peak a t 227.8 eV b i n d i n g energy. A g a i n Mo(IV) i s o b s e r v e d , b u t no molybdates were observed. T h i s r e d u c t i o n was observed t o proceed r a p i d l y a t f i r s t , so t h a t , a f t e r 6 h o u r s , the Mo(IV)/ Mo(VI) and Mo/Mo(VI) r a t i o s were 0.80 and 2.20, r e s p e c t i v e l y . Extended times ( t o 9 h o u r s ) d i d n o t g r e a t l y a f f e c t t h e s e r a t i o s . P a t t e r s o n and co workers ( 4) observed s i m i l a r b e h a v i o r f o r a s u p p o r t e d molybdenum c a t a l y s t , i n w h i c h case l i t t l e f u r t h e r r e d u c t i o n o f Mo(VI) t o Mo was observed a f t e r 7 hours i n hydrogen. I n t h i s c a s e , t h e presence o f 3


H AWN ET AL.



A. initial

239

235

231

227

Binding Energy

F i g u r e 2. Molybdenum (3d) XPS s p e c t r a o f a molybdenum on carbon c a t a l y s t f o l l o w i n g v a r i o u s t r e a t m e n t schemes; A. c a t a l y s t as p r e p a r e d ; B. f o l l o w i n g 500°C f o r 6 hours i n h e l i u m ; C. f o l l o w i n g 500°C f o r 6 hours i n hydrogen.



42

M o ( I V ) , and i t s o p t i m i z a t i o n on the c a t a l y s t s u r f a c e , i s an impor t a n t c o n s i d e r a t i o n i n the development o f t h i s c a t a l y s t , as Mo(IV) i s b e l i e v e d t o be the a c t i v e component f o r s e v e r a l c a t a l y t i c p r o c e s s e s (8).


2)

S i l i c o n Anode X-Ray Source A l t h o u g h XPS has been a p p l i e d w i t h success i n a wide v a r i e t y o f c a t a l y s t p r o b l e m s , c e r t a i n s i t u a t i o n s a r i s e where i n t e r f e r e n c e s o f photoelectron lines from d i f f e r e n t elements make the technique d i f f i c u l t t o use. An i n t e r e s t i n g example i s the copper/aluminum system, i n w h i c h case the copper (3s) and (3p) l i n e s i n t e r f e r e w i t h the aluminum(2s) and (2p) l i n e s ( F i g u r e 3 ) . These l i n e s cannot be i s o l a t e d from each o t h e r by u s i n g a d i f f e r e n t x - r a y l i n e , as i s commonly done w i t h Auger l i n e s o v e r l a p p i n g XPS p h o t o l i n e s . I n t h e s e s p e c i a l c a s e s , the b e s t s o l u t i o n i s t o employ a h i g h e r energy x - r a y s o u r c e t o a c c e s s deeper, c o r e - l e v e l t r a n s i t i o n s , f o r example the A l ( l s ) p h o t o l i n e i n the copper/aluminum system. S e v e r a l c o n s i d e r a t i o n s must be made when s e l e c t i n g an anode material. F o r r o u t i n e a n a l y t i c a l work, one anode o f a d u a l anode s o u r c e s h o u l d be magnesium because o f the narrow x - r a y l i n e w i d t h and e x t e n s i v e l i t e r a t u r e base. A n o t h e r c o n s i d e r a t i o n i s the energy range o f the a n a l y z e r , as p h o t o e l e c t i o n l i n e s w i t h k i n e t i c e n e r g i e s e x c e e d i n g the k i n e t i c energy range o f the a n a l y z e r would be i n a c c e s sible. T h e r e f o r e , i n the PHI 555 system w i t h an energy range o f 2400 e l e c t r o n v o l t s , g o l d (Ma=2122.9 e V ) , z i r c o n i u m (La=2042.4 e V ) , and s i l i c o n (Ka=1739.4 eV) would be p o s s i b l e c h o i c e s . C a s t l e and co-workers have d i s c u s s e d the use o f S i K a ( 9 ) and Z r L a ( 1 0 ) , w h i l e the use o f AuMa has been demonstrated by Wagner ( 1 1 ) . A n o t h e r c o n s i d e r a t i o n i s the n a t u r a l l i n e w i d t h and s a t e l l i t e s t r u c t u r e o f the x - r a y l i n e used. T i t a n i u m (TiKof=4510.9 eV) has seen l i m i t e d use (12) f o r n o n - d e s t r u c t i v e d e p t h p r o f i l i n g , b u t the o b s e r v e d s p e c t r a are c o m p l i c a t e d by the T i K a s a t e l l i t e s t r u c t u r e and the l a r g e n a t u r a l l i n e w i d t h o f 2.0 eV ( 1 3 ) . S i K a i s a good c h o i c e because o f i t s energy and narrow l i n e w i d t h o f 1.0 eV (9). F i g u r e 4 shows some o f the element core l e v e l s w h i c h can be e x c i t e d w i t h magnesium (Kcf=1253.6 e V ) , aluminum (Ka=l486.6 eV) and s i l i c o n (Ka=1739.4 eV) s o u r c e s . The h i g h c r o s s s e c t i o n o f 6.01 f o r the A l ( l s ) p h o t o l i n e , as measured r e l a t i v e t o the f l u o r i n e ( I s ) p h o t o l i n e u s i n g S i K a x - r a d i a t i o n , i s s u b s t a n t i a l l y g r e a t e r t h a n t h a t o f the A l ( 2 p ) p h o t o l i n e , a t 0.170. In a d d i t i o n , the r e s u l t i n g Auger t r a n s i t i o n s , such as the Al(KLL) s e r i e s , a l l o w the development o f Auger parameters f o r t h i s s e r i e s . F i g u r e 5 shows such an Auger parameter p l o t f o r a s e r i e s o f aluminum compounds. Due t o c r o w d i n g , s e v e r a l v a l u e s g i v e n i n T a b l e I are o m i t t e d from t h i s p l o t . Most o f the compounds are grouped t o the l o w e r l e f t , whereas aluminum m e t a l i s a t the upper r i g h t . I n t e r m e d i a t e between t h e s e are sodium z e o l i t e and z i n c a l u m i n a t e (ZnAl 0 ). A l t h o u g h the range covered by the A l ( l s ) l i n e f o r the compounds i s n e a r l y the same as t h a t f o r the A l ( 2 p ) l i n e (5.4 eV as compared t o 4.3 e V ) , the added d i m e n s i o n o f the Auger (KL23L23) l i n e a l l o w s d i f f e r e n t i a t i o n o f compounds not d i s t i n g u i s h a b l e u s i n g t h e i r A l ( 2 p ) l i n e s a l o n e ( f o r example, A1(0H)3 and A I C I 3 ) . B e r y l l u i m (7 m i c r o n ) , s i l i c o n d i o x i d e (10 m i c r o n ) , and aluminum (2 m i c r o n ) were t r i e d as x - r a y windows. B e r y l l u i m was the b e s t 2

4



HAWN ET A L .


133

129

88

84

125 121 Binding Energy

117

113

80

72

68

76

Binding Energy

F i g u r e 3. XPS s p e c t r a o f a l u m i n a - s u p p o r t e d copper c a t a l y s t , showing i n t e r f e r e n c e o f XPS p h o t o l i n e s : A. copper (3s) and aluminum ( 2 s ) r e g i o n s ; B. copper (3p) and aluminum (2p) regions.



Relative Intensity


1800 Si Κ α 1600 •Br(2p )

Al(1s)

3/2

ΑΙ Κ α 1400 Mg(1s) Mg Κ α 1200 Nads) 1000

-o

800

600

400

AI(KLL) "ir(LMM)

Mg(KLL)

Na(KLL)

200 Al(2s) Al(2p) ' Br(3d)

Mg(2s) Mg(2p)

Na(2s) Na(2p)

F i g u r e 4. P o s i t i o n s and r e l a t i v e i n t e n s i t i e s o f XPS and Auger transitions using s i l i c o n , aluminum, and magnesium x-ray sources.



4.

HAWN ET AL.

-1566

45


-1564

-1562

-1560

-1558

Al(1s) Binding Energy

F i g u r e 5. Auger c h e m i c a l s t a t e p l o t f o r aluminum compounds as o b t a i n e d u s i n g s i l i c o n Κα x - r a d i a t i o n .

Table I .

C h e m i c a l S t a t e Data F o r V a r i o u s Aluminum Compounds Ob t a i n e d U s i n g S i l i c o n Κα X - R a d i a t i o n (1739.4)

Compound

Al(ls)

Al V-AI2O3 A1 0 A1(0H) A1(P0 ) A1(N0 ) Al C l Al I Al F Na(A)Zeolite ZnAl 0 1:1 Cu/Al

1558.3 1562.4 1561.3 1562.1 1562.8 1562.0 1562.3 1561.4 1563.7 1561.5 1561.6 1562.1

2

3

3

4

3

3

3

3

2

4

3

Al(KL

2 3

L 3) 2

364.1 353.2 353.8 353.4 354.7 353.8 354.1 354.5 355.3 353.6 351.6 353.5

N E

Al(2p)*

AUKL23L23W 1393.3 1386.2 1386.7 1386.0 1384.7 1385.7 1385.4 1385.0 1384.1 1385.9 1387.8 1386.0

2951.6 2948.6 2948.0 2948.1 2947.5 2947.7 2947.7 2946.4 2947.8 2947.4 2949.4 2948.1

72.2 75.3 74.7 74.9 75.8 75.2 75.8 76.0 77.0 74.4 74.7

--

" U s i n g Mg Κα r a d i a t i o n B i n d i n g energy r e f e r e n c e d t o A u ( 4 f 7/2) l e v e l a t 83.8 eV f o r t h i n l a y e r o f g o l d e v a p o r a t e d onto sample.



46

c h o i c e s i n c e t h e h i g h e s t count r a t e s were o b s e r v e d , p r o d u c i n g Ag(3d 5/2) count r a t e s 26 p e r c e n t t h a t o f t h e magnesium anode w i t h a 2μπι aluminuim window a t t h e same a n a l y z e r pass energy. S i l i c o n d i o x i d e and aluminum windows lowered t h e observed count r a t e s on s i l v e r t o 4 p e r c e n t o f t h a t o f magnesium. When u s i n g aluminum as t h e window m a t e r i a l , ghost l i n e s due t o copper were 25 p e r c e n t o f t h e i n t e n s i t y o f t h e Ag(3d 5/2) p h o t o l i n e . Some c r a c k i n g o f t h e S i 0 window was o b s e r v e d w i t h u s e , presumably due t o h e a t d i s t o r t i o n . The n a r r o w e s t f u l l - w i d t h - a t - h a l f - m a x i m u m peak h e i g h t (FWHM) f o r Ag(3d 5/2) was 1.36 eV, i n w h i c h case some assymetry o f t h e l i n e was o b s e r v e d due t o s e p a r a t i o n o f t h e SiKu"i,2 components. T h i s was n o t a problem i n r o u t i n e use. The Ag (3d 5/2) FWHM o b s e r v e d u s i n g t h e same a n a l y z e r pass energy and magnesium Κα x - r a d i a t i o n was 0.96 eV.


2

3)

A p p l i c a t i o n o f XPS U s i n g S i l i c o n Anode X-Ray S o u r c e , S c a n n i n g Auger M i c r o p r o b e , and R e a c t i o n F a c i l i t y i n a Copper/Aluminum C a t a l y s t System The use o f t h i s anode i n c a t a l y s t a n a l y s i s was demonstrated on a commercially a v a i l a b l e p e l l e t i z e d c a t a l y s t with a bulk composition o f 84 w e i g h t p e r c e n t CuO, 14% A 1 0 , 1% N a 0 , and 1% g r a p h i t e b i n d e r . T h i s c a t a l y s t has t h e form o f h a r d g l o s s y p e l l e t s a p p r o x i m a t e l y 3 mm i n d i a m e t e r by 3 mm i n l e n g t h . The c a t a l y s t i s w i d e l y used i n t h e reduced form f o r h y d r o l y s i s o f p r i m a r y amines. P e r i o d i c regenera t i o n o f t h i s c a t a l y s t involves m i l d r e o x i d a t i o n t o burn o f f r e s i d u a l h y d r o c a r b o n s , f o l l o w e d by r e - r e d u c t i o n . The g o a l o f t h i s work was t o u n d e r s t a n d t h e s u r f a c e c o m p o s i t i o n a l changes w h i c h o c c u r r e d during repeated regenerations. R e f e r r i n g a g a i n t o F i g u r e 3, i t was i m p o s s i b l e t o d i s c e r n t h e r o l e o f aluminum i n t h i s c a t a l y s t due t o t h e i n t e r f e r e n c e o f copper p h o t o l i n e s w i t h t h e A l ( 2 s ) and A l ( 2 p ) photolines. F i g u r e 6 compares t h e XPS s u r v e y scans o b t a i n e d from t h e a s - r e c e i v e d m a t e r i a l , and a f t e r r e d u c t i o n i n a hydrogen/helium gas m i x t u r e a t 200°C f o r 12 h o u r s . B o t h s p e c t r a were o b t a i n e d u s i n g SiKa x - r a d i a t i o n . The most n o t a b l e d i f f e r e n c e s between t h e s e two s p e c t r a a r e t h e i n c r e a s e d i n t e n s i t y o f t h e A l ( l s ) and N a ( l s ) p h o t o l i n e s , and t h e l o s s o f s a t e l l i t e s t r u c t u r e i n t h e copper (2p) r e g i o n due t o r e d u c t i o n o f C u ( I I ) s p e c i e s t o C u ( I ) o r Cu m e t a l . The l a r g e c a r b o n ( I s ) i n t e n s i t y was s u r p r i s i n g c o n s i d e r i n g t h e low l e v e l o f g r a p h i t e added as b i n d e r . That carbon i s s e g r e g a t e d t o t h e p e l l e t s u r f a c e i s c l e a r l y i n d i c a t e d by comparison o f t h e c a r b o n ( I s ) / aluminum ( I s ) a t o m i c r a t i o s i n rows A o r C-F t o rows Β o r G i n T a b l e I I . I n t h i s c a s e , t h e powdered m a t e r i a l i s i n t e n d e d t o be representative of the bulk c a t a l y s t . These powders were d u s t e d onto c o n d u c t i v e tape f o r a n a l y s i s , and t h e r e f o r e t h e carbon-to-aluminum r a t i o s may be s l i g h t l y i n e r r o r due t o t h e sample p r e p a r a t i o n . The e x c e p t i o n a l l y h i g h C ( l s ) / A l ( l s ) r a t i o f o r t h e p e l l e t i z e d s t a r t i n g m a t e r i a l r e s u l t s from t h e p r e s e n c e o f s u r f a c e hydrocarbons d e p o s i t e d d u r i n g p r o c e s s i n g o r subsequent h a n d l i n g . The carbon ( I s ) peak was v e r y b r o a d due t o t h i s c o n t r i b u t i o n a t 284.6 eV b i n d i n g energy, as w e l l as g r a p h i t i c type carbon a t 284.3 eV. The carbon ( I s ) peak f o r t h e powdered s t a r t i n g m a t e r i a l was n a r r o w e r due t o t h e reduced c o n t r i b u t i o n o f t h i s 284.6 eV component t o t h e spectrum. F o l l o w i n g r e d u c t i o n and o x i d a t i o n , t h e carbon was p r e d o m i n a n t l y g r a p h i t i c i n n a t u r e , i n w h i c h case t h e h y d r o c a r b o n contaminants were q u i c k l y burned o f f d u r i n g t r e a t m e n t . A l s o note t h e s u b s t a n t i a l 2

3

2


HAWN ET A L .



A. Cu/AI, initial

1600

1400

1200

1000

800

600

400

200

0

Binding Energy

F i g u r e 6. XPS wide scans o f a commercial copper/aluminum e x t r u d e d c a t a l y s t o b t a i n e d u s i n g s i l i c o n Κα x - r a d i a t i o n : A. u n t r e a t e d c a t a l y s t ; B. f o l l o w i n g 200°C f o r 12 hours i n 1 0 % hydrogen/90% h e l i u m gas m i x t u r e .

American Chemical Society Library 1155 16th St., N.W.

In Catalyst Characterization Science; Deviney, M., et al.; Washington, ACS Symposium Series; American Chemical D.C. Society:20036 Washington, DC, 1985.



48

T a b l e I I . E l e m e n t a l Atomic R a t i o s f o r Copper/Aluminum C a t a l y s t F o l l o w i n g V a r i o u s Treatments as Obtained U s i n g S i l i c o n Κα X-Radiation

Condition A Starting Material Β Starting Material C (R) D (R),(0) Ε (R),(0), F

00,(0)

(R),(0),(R) (0),(R) G Same as F

Cu(2p 3/2) Al(ls)

Na(ls) Al(ls)

0(ls) Al(ls)

C(ls) Al(ls)

Pellet

2.0

0.9

9.6

28.2

Powdered Pellet Pellet

1.0 0.6 1.6

0.3 1.8 3.1

3.7 3.2 6.7

4.8 10.7 10.3

Pellet

1.5

2.3

5.0

9.5

Pellet Powdered

0.4 0.5

2.2 0.3

2.1 2.4

5.1 2.3

P r e p a r a t i o n Code: (R) 12 hours a t 200°C i n 10% H /He (0) 12 hours a t 200°C i n 10% 0 /He 2

2



4.

HAWN ET AL.


49

i n c r e a s e i n sodium a t the p e l l e t s u r f a c e f o l l o w i n g r e d u c t i o n and o x i d a t i o n , d e m o n s t r a t i n g the m o b i l i t y o f sodium, e s p e c i a l l y under reducing conditions. The lower atomic r a t i o s f o r t h e o t h e r compo nents i n the c a t a l y s t r e s u l t i n p a r t from s u r f a c e coverage by sodium as d i s c u s s e d l a t e r . H i g h r e s o l u t i o n s c a n n i n g Auger microprobe (SAM) a n a l y s i s i n d i c a t e d t h a t the h i g h s u r f a c e carbon was r e l a t e d t o the presence o f exposed g r a p h i t e i s l a n d s on the s u r f a c e o f the c a t a l y s t p e l l e t s . F i g u r e 7A shows a secondary e l e c t r o n (SEM) p h o t o m i c r o g r a p h o f t h e s u r f a c e o f the u n t r e a t e d c a t a l y s t . P o i n t mode Auger s p e c t r a o b t a i n e d from the d a r k e r a r e a s i n t h i s p h o t o m i c r o g r a p h showed o n l y carbon t o be p r e s e n t . I n comparison, s p e c t r a o b t a i n e d from t h e grey a r e a s e v i d e n t i n t h i s p h o t o m i c r o g r a p h showed t h e presence o f c a r b o n , oxygen, copper, sodium, and aluminum. F i g u r e 7B shows t h e carbon (KLL) Auger map c o r r e s p o n d i n g t o the a r e a shown i n 7A. Note the correspondence o f dark areas i n the SEM image t o t h e h i g h carbon c o n t e n t areas e v i d e n t i n t h e Auger map. F o l l o w i n g i n i t i a l r e d u c t i o n a t 200°C, photomicrographs showed the development o f d i s t i n c t l i g h t - c o l o r e d i s l a n d s on t h e c a t a l y s t s u r f a c e , i n a d d i t i o n t o t h e d a r k (carbon) i s l a n d s and grey a r e a s p r e v i o u s l y observed ( F i g u r e 8A). P o i n t mode s p e c t r a (8 kV, 10 nanoamp p r i m a r y beam) o b t a i n e d i n t h e s e l i g h t areas i n d i c a t e d p r i m a r i l y copper and oxygen t o be p r e s ent, s i m i l a r t o p o i n t mode s p e c t r a o b t a i n e d from g r e y areas i n t h e p h o t o m i c r o g r a p h . Auger s p e c t r a a c q u i r e d from the e n t i r e a r e a imaged i n F i g u r e 8A i n d i c a t e d t h a t sodium and oxygen were t h e main s u r f a c e components. The sodium (KLL) Auger map o b t a i n e d i n the (peakbackground) /background mode c o r r e s p o n d i n g t o F i g u r e 8A i s shown i n F i g u r e 8B. Note the correspondence o f the l i g h t c o l o r e d a r e a s i n the SEM image t o h i g h sodium a r e a s observed i n F i g u r e 8B. The oxygen (KLL) Auger map ( F i g u r e 8) shows s i g n i f i c a n t c o r r e l a t i o n w i t h t h e sodium Auger map, s u g g e s t i n g t h a t t h e s e l i g h t c o l o r e d a r e a s observed i n F i g u r e 8A are sodium o x i d e ( N a 0 ) . The c o n f l i c t between n i l sodium observed i n t h e p o i n t mode i n l i g h t a r e a s o f t h e SEM p h o t o m i c r o g r a p h , and s u b s t a n t i a l sodium i n the sodium maps i n t h e s e a r e a s , p o i n t s t o t h e m i g r a t i o n o f sodium caused by the p r i m a r y e l e c t r o n beam. Even though the p r i m a r y beam c u r r e n t s used here were low (10 n A ) , the beam c u r r e n t d e n s i t y i n the p o i n t mode was h i g h ( a p p r o x i m a t e l y 0.5 amps p e r square c e n t i m e t e r ) . D e r e a l i z a t i o n o f the p r i m a r y beam by r a s t e r i n g , o r by d e f l e c t i o n as i s done d u r i n g mapping, produced s u f f i c i e n t l y lower beam c u r r e n t d e n s i t y , thus a l l e v i a t i n g sodium m i g r a t i o n . The i s l a n d i n g o f sodium o x i d e as observed i n t h i s case i s most l i k e l y caused by t h e i n a b i l i t y o f sodium o x i d e t o wet the reduced copper s u r f a c e . These i s l a n d s were observed o n l y d u r i n g the i n i t i a l r e d u c t i o n s t e p ; a u n i f o r m s u r f a c e d i s t r i b u t i o n o f sodium was ob s e r v e d f o l l o w i n g subsequent o x i d a t i o n , i n w h i c h case t h e o x i d e n a t u r e o f s u r f a c e would be more w e t t a b l e . R e f e r r i n g a g a i n t o T a b l e I I and the XPS r e s u l t s , the h i g h N a ( l s ) / A l ( l s ) atomic r a t i o observed f o l l o w i n g r e d u c t i o n and o x i d a t i o n (row D) i s a r e s u l t o f more u n i f o r m coverage o f the s u r f a c e by sodium. When r u n n i n g h y d r o l y s i s r e a c t i o n s i n the l i q u i d phase, i t was observed t h a t the sodium c o u l d be washed from the bed by f l u s h i n g w i t h water and m o n i t o r i n g the change i n pH o f the bed e f f l u e n t . I f t h i s p r e c a u t i o n was not o b s e r v e d , i n i t i a l low y i e l d s o f p r o d u c t were observed u n t i l sodium was e n t i r e l y removed from the bed. 2




50

F i g u r e 7. Secondary e l e c t r o n m i c r o g r a p h and carbon (KLL) map of untreated c a t a l y s t : A. p h o t o m i c r o g r a p h o b t a i n e d a t 8kV, O.lnA p r i m a r y beam; B. c o r r e s p o n d i n g carbon (KLL) Auger map o b t a i n e d i n (peak-background)/ background mode, a t 8kv, lOnA p r i m a r y beam.




HAWN ET A L .

F i g u r e 8. Secondary e l e c t r o n p h o t o m i c r o g r a p h , sodium ( K L L ) , and oxygen (KLL) Auger maps o f c a t a l y s t f o l l o w i n g s i n g l e r e d u c t i o n : A. SEM p h o t o m i c r o g r a p h o b t a i n e d w i t h 8 k v , 0.1 nA p r i m a r y beam; B. c o r r e s p o n d i n g sodium (KLL) auger map o b t a i n e d i n (peak-background) /background mode; C. c o r r e s p o n d i n g oxygen (KLL) Auger map o b t a i n e d i n (peak-background )/background mode w i t h 8kv, ΙΟηΑ p r i m a r y beam. In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.



52

Photomicrographs o b t a i n e d f o l l o w i n g the second o x i d a t i o n showed the appearance o f s m a l l c r a c k s throughout the p e l l e t s . In a c t u a l use, these c r a c k s would be d e t r i m e n t a l s i n c e they are the p r e c u r s o r to p e l l e t f r a c t u r e and subsequent compaction, w h i c h would r e s u l t i n i n c r e a s e d p r e s s u r e drop a c r o s s the r e a c t o r . E x a m i n a t i o n o f aluminum ( I s ) and copper (2p) s p e c t r a suggest t h a t a strong metal-support i n t e r a c t i o n occurs i n t h i s c a t a l y s t . Aluminum ( I s ) s p e c t r a o b t a i n e d a f t e r v a r i o u s t r e a t m e n t s are shown i n F i g u r e s 9 and 10. The i n i t i a l c a t a l y s t A l ( l s ) spectrum i s composed o f two peaks, w i t h the h i g h e r b i n d i n g energy component a t 1562.4 eV c o n s i s t e n t w i t h aluminum o x i d e . The lower b i n d i n g energy component i s c e n t e r e d a t 1560.4 eV, w h i l e the A l ( K L L ) t r a n s i t i o n i s l o c a t e d a t 352.2 eV b i n d i n g energy. F o l l o w i n g repeated r e d u c t i o n and o x i d a tion, two a d d i t i o n a l components appear i n the aluminum ( I s ) spectrum, a t b i n d i n g e n e r g i e s o f 1558.0 eV and 1555.6 eV. Although the 1558.0 eV component i s i n d i c a t i v e o f aluminum m e t a l , aluminum m e t a l i s not e x p e c t e d under o x i d a t i v e c o n d i t i o n s , as i n F i g u r e 9C. I n a d d i t i o n , the plasmon l i n e s a s s o c i a t e d w i t h the A l ( l s ) l i n e , s i m i l a r t o those observed f o r the A l ( 2 s ) l i n e , are not observed i n t h i s case. T h i s s h o u l d be compared t o the A l ( l s ) spectrum f o r the powdered c a t a l y s t , shown i n F i g u r e 10C, w h i c h shows a s i n g l e component a t 1562.4 eV. The A l ( K L L ) Auger t r a n s i t i o n was observed a t 353.3 eV b i n d i n g energy, s u g g e s t i n g t h a t the b u l k aluminum i s p r e s e n t as an aluminum o x i d e ( y - A l 0 ) . As i n the powdered c a t a l y s t , no changes i n e i t h e r the A l ( l s ) o r A l ( K L L ) Auger s p e c t r a were observed f o r p e l l e t s w h i c h were c l e a v e d and s u b j e c t e d t o the same t r e a t m e n t c y c l e s , so t h a t A 1 0 was the o n l y aluminum compound observed. The copper (2p) s p e c t r a f o r t h i s c a t a l y s t o b t a i n e d w i t h mag nesium Κα x - r a d i a t i o n showed similar unusual behavior, as i l l u s t r a t e d i n F i g u r e 11. Whereas the powdered m a t e r i a l showed o n l y C u ( I I ) o x i d e t o be p r e s e n t f o l l o w i n g o x i d a t i o n and copper m e t a l f o l l o w i n g r e d u c t i o n , the s u r f a c e e x h i b i t e d p r e d o m i n a t l y copper m e t a l f o l l o w i n g r e d u c t i o n , and two copper components a t 932.4 eV and 934.8 eV e i t h e r i n i t i a l l y o r f o l l o w i n g o x i d a t i o n . The 934.8 eV component i s a t t r i b u t e d t o copper ( I I ) c a r b o n a t e , and i s supported by the p r e s e n c e of an a d d i t i o n a l peak i n the carbon ( I s ) spectrum a t 289.6 eV b i n d i n g energy, i n d i c a t i v e of carbonate. The 932.4 eV copper peak, a l o n g w i t h the copper (LMM) auger t r a n s i t i o n a t 335.9 eV b i n d i n g energy, suggests a copper ( I ) s p e c i e s . T h i s i s a l s o sup p o r t e d by the low i n t e n s i t y o f the shake-up l i n e s r e l a t i v e t o the Cu(2p) l i n e s . Note p a r t i c u l a r l y t h a t t h i s b e h a v i o r was not observed i n powder m a t e r i a l o r i n c l e a v e d p e l l e t s s u b j e c t e d t o the same treatments. S i m i l a r experiments w i t h copper d i s p e r s e d on A 1 0 d i d not show any u n u s u a l b e h a v i o r o f the A l ( l s ) o r Cu(2p) p h o t o l i n e s . In t h i s c a s e , the copper c o u l d be e a s i l y c y c l e d between CuO under o x i d a t i v e c o n d i t i o n s , t o Cu m e t a l d u r i n g r e d u c i n g c o n d i t i o n s . We observed o n l y a s l i g h t s h i f t (

Application of Surface Analysis Techniques in the Study of Catalyst

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