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ZnO Catalysts: IR, VIS, UV Study of CO Chemisorption
M. Che and G.C. Bond (Editors),Adsorption and Catalysis on Omde Surfaces 0 1985 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
23 5
SURFACE C H A R A C T E R I Z A T I O N OF Cu/ZnO CATALYSTS: I R , V I S , U V STUDY OF CO CHEMISORPTION
G. G H I O T T I , F. BOCCUZZI and A. C H I O R I N O I s t i t u t o d i Chimica F i s i c a , C.so
M. D ’ A z e g l i o 48, 10125 T o r i n o ( I t a l y )
ABSTRACT
The s t a t e o f copper i n Cu/ZnO c a t a l y s t s a t d i f f e r e n t r e d u c t i o n stages were s t u d i e d , m a i n l y by c h e m i s o r p t i o n o f CO a t 77 K. The s p e c t r o s c o p i c d a t a o f f u l l y o x i d i z e d samples i n d i c a t e t h a t a l a r g e p a r t o f copper i o n s can be r e a d i l y reduced t o Cu(o) o r Cu(1) by CO a t 77 K . Two d i f f e r e n t Cu(1)CO complexes a r e p r e sent, a b s o r b i n g a t 2130 and 2110 cm-’ r e s p e c t i v e l y . C u p r i c i o n s s t r o n g l y r e s i s t a n t t o t h e r e d u c t i o n g i v e r i s e t o Cu(1I)CO complex a b s o r b i n g a t 2172 cni’unstab l e i n vacuum. C a r b o n a t e - l i k e species appear as o x i d a t i o n p r o d u c t s , m a i n l y adsorbed on Z n ( I 1 ) and C u ( I 1 ) i o n s . On f u l l y reduced samples two chemisorbed spec i e s a r e p r e s e n t . One a b s o r b i n g a t 2098 cm-l, i s assigned t o CO chemisorbed on h i g h i n d e x m i c r o f a c e t s o f m e t a l p a r t i c l e s . The o t h e r , a b s o r b i n g a t 2070 cm-’, i s assigned t o CO chemisorbed on edges and c o r n e r s o f metal p a r t i c l e s .
INTROOUCTION A f t e r t h e i n t r o d u c t i o n o f Cu/ZnO/Al 0 and Cu/ZnO/Cr203 c a t a l y s t s f o r t h e low 2 3 p r e s s u r e methanol s y n t h e s i s and low temperature w a t e r s h i f t r e a c t i o n , many works ( r e f . 1) have e s t a b l i s h e d t h a t t h e h i g h a c t i v i t y r e s i d e s i n t h e b i n a r y copperz i n c o x i d e system. The aim o f t h i s work i s t o b r i n g a c o n t r i b u t i o n t o t h e knowl e d g e o f t h e f a c t o r s which c o n t r i b u t e t o t h e a c t i v i t y o f t h i s b i n a r y system, s t a r t i n g f r o m a p r e c u r s o r c o n s t i t u t e d by a s i n g l e phase Cu Zn mixed b a s i c carbona t e .
EXPERIMENTAL The c a t a l y s t p r e p a r a t i o n The s i n g l e phase p r e c u r s o r corresponds t o a s o l i d s o l u t i o n o f copper i n t h e h y d r o z i n c i t e s t r u c t u r e (Cu, Zn) ( C O ) (OH) p r e p a r e d and c h a r a c t e r i z e d by P e t r i 5 3 2 6 n i e t a l . ( r e f . 2 ) . Samples w i t h d i f f e r e n t Cu c o n t e n t were prepared. They a r e id e n t i f i e d by symbols Cx, where x r e p r e s e n t s t h e Cu/(CutZn) a t o m i c percentage. S i n g l e phase samples were c a l c i n e d a t 588 K and c h a r a c t e r i z e d by P e t r i n i e t a l .
236
( r e f . 2) w i t h XRO p a t t e r n s . C a l c i n e d samples examined, C3, C5 and C10
showed
t h e XRO p a t t e r n s o f ZnO and CuO w i t h i n t e n s i t y p r o p o r t i o n a l t o t h e c a t i o n concent r a t i o n s . C r i s t a l l i t e s i z e s determined by t h e S h e r r e r f o r m u l a were 15-20 nm and 6-8 nm f o r ZnO and CuO r e s p e c t i v e l y . C a l c i n e d samples powdered o r compressed i n p e l l e t s were s u b m i t t e d t o a l t e r n a t e t r e a t m e n t s i n vacuo and i n p u r e oxygen s l o w l y i n c r e a s i n g temperature f r o m 298 t o 588
K. We name samples
so t r e a t e d f u l l y o x i d i z e d samples.
2 The s p e c i f i c s u r f a c e areas o f f u l l y o x i d i z e d samples a r e 50, 40 and 40 m / g r f o r
C3, C5 and C10 r e s p e c t i v e l y . Reduced samples were prepared w i t h t h e f o l l o w i n g processes: R1) r e d u c t i o n i n a i n N m i x t u r e i n c r e a s i n g temperature up t o 453 K; R2) r e d u c t i o n i n 0 , 5 2 2 3% H i n N m i x t u r e s i n c r e a s i n g temperature up t o 503 K; R3) r e d u c t i o n i n 0.5-3% 2 2 H i n N m i x t u r e s and i n p u r e H i n c r e a s i n g temperature up t o 503 K . 2 2 2 The H consumption by TPR ( r e f . 2) i n d i c a t e s t h e complete r e d u c t i o n o f c u p r i c 2 o x i d e t o m e t a l l i c copper b o t h i n R2 and R3 processes.
0.5% H
Copper p a r t i c l e s were n o t d e t e c t a b l e by XRO o r TEM a n a l y s i s .
Experimental t e c h n i q u e s The I R s p e c t r a were r e c o r d e d w i t h a Perkin-Elmer 580 B i n t e r f a c e d w i t h
a
3600 Data S t a t i o n . The p e l l e t was mounted i n a s u i t a b l e I R c e l l which a l l o w e d b o t h h e a t i n g and cool i n g (down t o 77 surements a t 77 p r o x i m a t e l y 7-10
K) i n s i t u under vacuum o r c o n t r o l l e d atmosphere. D u r i n g I R meaK some h e a t i n g occurs a t t h e c e n t e r of t h e p e l l e t , v a l u a b l e apK. The U V - V I S - N I R d i f f u s e r e f l e c t a n c e
s p e c t r a were r e c o r d e d
by u s i n g a V a r i a n 2390 spectrophotometer equipped w i t h a d i f f u s e r e f l e c t a n c e sphere and i n t e r f a c e d w i t h an Apple I 1 p l u s microcomputer. M i c r o g r a v i m e t r i c measurements were c a r r i e d o u t w i t h a S a r t o r i u s 4102 e l e c t r o b a l a n c e o p e r a t i n g i n vacuo and i n c o n t r o l l e d atmosphere.
RESULTS I R s p e c t r a and m i c r o g r a v i m e t r i c measurements o f CO a d s o r p t i o n A d s o r p t i o n a t 77 K on f u l l y o x i d i z e d C3 samples. When CO i s adsorbed a t 77 K s p e c t r a l changes occur i n t h e h y d r o x y l s t r e t c h i n g r e g i o n (3800-3000 cm-I), d r o x y l bending and s u r f a c e mode ( r e f . 3 ) r e g i o n (800-6C0 cm-’), r e g i o n (1600-800 cm-l),
hy-
carbonate mode
and c a r b o n i l i c s p e c i e s r e g i o n (2000-2300 cm-’).
A typi-
231
cal experiment i s i l l u s t r a t e d in Fig. 1 , where only the 2300-2000 crn-' a n d the 1600-800 cm-l regions a r e reported.
O.D.
f
It\ t
', ,2115
1.o.
1575,
1330
0.5
2100
2ibo
1660
Fig. 1 . CO adsorption a t 77 K on a C3 f u l l y oxidized sample ( s o l i d l i n e ) and successive desorption a t increasing temperatures ( o t h e r symbols, see in t h e t e x t ) . Solid curve r e f e r s t o bands immediately appearing when the sample i s contacted with 25 Torr of CO. I n the 2300-2000 cm-l range a broad (FWHM = 80 cm-') and intense absorption appears with maximum a t 2115 cm
-1
and two sharper components a t
2152 and 2178 cm-l. I n t h e 1600-800 cm-l range four complexabsorptions a r e pre-1 sent: two more important a t 1570-1520 cm-l and a t 1340-1320 cm , and two l e s s important a t 1047-1015 cm-l and a t 856-850 cm-l ( t h i s l a s t not reported in figure). The band a t 2152 cm-l disappears f i r s t when t h e gas phase i s evacuated, simultaneouslythe surface hydroxyl s t r e t c h i n g and bending modes a r e completely restored in t h e i r original positions and half widths. The shoulder a t 2178 cm-l gradually
238
decreases showing a b l u e - s h i f t and disappears a f t e r 15 min of evacuation (dotted curve). Other
b a n d s a r e unaffected by desorption a t 77 K .
Further modifications a r e v i s i b l e only by desorption a t higher temperature. Dashed curve in Fig. 1 represents the evolution of the spectrum a f t e r 1 5 min of evacuation a t 298 K . The broad absorption a t 2115 cm-l i s now sharper, increased in i n t e n s i t y and s h i f t e d t o higher frequency ( a component a t 2098 cm-’ has been eroded, one a t 2130 cm-l i n c r e a s e d ) . A t the same time the complex absorptions in the 1600-800 cm-’ range grow u p . A prolonged outgassing a t 298 K f i n a l l y decreases the 2130 cm-l band while some increments in the carbonate modes s t i l l occur.
A t 373 K the 2130 cm-’ band disappears (black c i r c l e curve). Other bands remarkably decrease only by desorption a t 573 K (dot-dashed curve). After t h i s complete desorption the sample i s cooled down again t o 77 K and CO i s readmitted. I n Fig. 2 t h e recorded spectrum ( s o l i d l i n e ) i s compared with t h a t obtained a f t e r the f i r s t i n t e r a c t i o n (dashed l i n e ) .
F i g . 2. Comparison between spectra o f CO adsorbed a t 77 K on: a C3 f u l l y oxidized sample (dotted l i n e ) ; and the same C3 sample which a f t e r the f i r s t CO adsorption has been completely cleaned by evacuation a t 573 K ( s o l i d l i n e ) .
239
The most e v i d e n t d i f f e r e n c e s a r e : i)no bands appear i n t h e 1600-800 cm-l r e g i o n -1 ( n o t r e p o r t e d i n f i g u r e ) ; i i ) i n t h e 2300-2000 cm range t h e i n t e n s e and broad a b s o r p t i o n i s now more s t r u c t u r a t e d , t h r e e components a r e e v i d e n t , one a t 2098 -1 cm , another, t h e most i n t e n s e one, a t 2110 cm-l, and f i n a l l y one v e r y weak a t -1 2130 cm A l l t h e s e components a r e i r r e v e r s i b l e a t 77 K. A t h i g h e r f r e q u e n c i e s
.
s h a r p e r components a r e p r e s e n t a t 2152, ( n o t e v i d e n t a t t h e r e p o r t e d coverage, = 0.5 T o r r ) , 2172 and 2178 cm-’, a l l r e v e r s i b l e f o r e v a c u a t i o n a t 77 K i n pco 2178 cm-l. The l a s t one shows a b l u e s h i f t t i l l 2193 t h e sequence 2152 > 2172 -1 cm
.
M i c r o g r a v i m e t r i c measurements performed on samples s o t r e a t e d show t h a t t h e r a t i o between t h e number o f CO molecules i r r e v e r s i b l y adsorbed a t 77 K and t h e t o t a l numbers o f copper atoms c o n t a i n e d i n t h e sample i s 0.32-0.34. Samples c o m p l e t e l y c l e a n e d by e v a c u a t i o n a t 573 K and c o o l e d t o 298 K l o s e 0.450.5% o f t h e i r w e i g h t . I f we i n t e r p r e t t h e w e i g h t l o s s as c o m p l e t e l y due t o oxygen l o s t f o r r e d u c t i o n , t h e r a t i o between t h e number o f l o s t oxygen atoms and readsorbed a t 2 i s such t h a t t h e r a t i o between t h e number o f taken oxygen atoms and t h e t o -
t h e t o t a l number o f copper atoms i s 0.11:0.13. 298
K
The w e i g h t o f 0
t a l number o f copper atoms i s 0.11. A d s o r p t i o n o f CO a t 77 K on t h e s e r e o x i d i z e d samples g i v e s r i s e t o s p e c t r a and g r a v i m e t r i c d a t a p r a c t i c a l l y c o i n c i d e n t w i t h those observed a f t e r f i r s t CO c o n t a c t on f u l l y o x i d i z e d samples. A d s o r p t i o n a t 298
K on f u l l y o x i d i z e d C3 samples. When CO (10 T o r r ) i s adsor-
bed a t 298 K on f u l l y o x i d i z e d samples, s p e c t r a l changes a r e v e r y s i m i l a r t o those observed f o r t h e i n t e r a c t i o n a t 77 K, b u t no m o d i f i c a t i o n s a r e now v i s i b l e i n t h e h y d r o x y l modes. I n t h e 2300-2000 cm-l range t h e broad a b s o r p t i o n w i t h maxi-
mum a t 2110 cm-’ appears w h i l e t h e h i g h e r f r e q u e n c y bands a r e m i s s i n g . I n t h e 1600-1800 cm-l range t h e f o u r complex a b s o r p t i o n s a r e p r e s e n t . The i n t e n s i t i e s o f a l l bands a r e now h i g h e r t h a n those observed f o r t h e i n t e r a c t i o n a t 77 K . A d s o r p t i o n a t 77 K and 298 K on f u l l y o x i d i z e d C5 and C10 samples. The e x p e r i ments p r e v i o u s l y d e s c r i b e d f o r C3 samples, p e r f o r m e d on C5 and C10 samples l e a d t o s i m i l a r r e s u l t s . The most e v i d e n t d i f f e r e n c e s a r e : i)t h e r a t i o between t h e i n t e g r a t e d i n t e n s i t y of t h e carbonate
-
l i k e s p e c i e s modes and t h a t o f t h e carbo-
n i l i c species decreases w i t h i n c r e a s i n g copper c o n t e n t ; i i ) spectrum o f gaseous C02 i s now v i s i b l e when i n t e r a c t i o n i s performed a t 298 K; i i i ) t h e bands o f t h e c a r b o n i l i c species show now a h i g h e r r e v e r s i b i l i t y t o t h e e v a c u a t i o n a t 298 K.
240
Adsorption a t 77 K on reduced samples. When CO i s adsorbed a t 77 K on samples (C3, C5, C10) reduced with the R1 treatment, no bands in the 1600-800 cm-’ range appear. Spectral changes occur i n a l l the other examined regions. I n the 2300-1 2000 cm range the broader and intense absorption a t lower frequency has been s u b s t i t u t e d by two new bands: one sharp (FWHM t y a t 2103 cm-’ and one broader (FWHM
=
40
=
ern-')
10 cm-’) a n d o f medium i n t e n s i at
.?i
2070 cm-’ .They a r e s t a b l e
under vacuum a t 77 K, unstable a t 298 K. Bands a t 2152, 2172 a n d 2178 cm-’ a r e s t i l l present, with t h e same f e a t u r e s described f o r oxidized samples. When CO i s adsorbed a t 77 K on R2 o r R3-treated samples the two bands, now a t -1 band 2098 and x 2070 cm-l, a r e present with increased i n t e n s i t y . The 2 1 7 2 cm i s completely missing, the 2178 cm-l one i s s t i l l present. Fig. 3 a shows a comparison between C O adsorbed on a C3 R1-treated sample (dotted l i n e ) a n d on a C3 R3-treated sample ( s o l i d l i n e ) a t a CO equilibrium pressure o f 0 . 5 Torr. Clean reduced samples a r e contacted with oxygen a t 77 K, evacuated, then CO i s adsorbed a t the same temperature. Fig. 3 b shows a comparison between CO spectra
O.D.
a
b 21.72
1.0-
,”\ f
2Q90
2098
0.5-
I
2200
I
2100
I
2200
2 l b
31cm“
Fig. 3. CO adsorption a t 77 K on C3 reduced samples. a ) Dotted l i n e , R1-treated sample; s o l i d l i n e , RP-treated sample. b ) Solid l i n e , R2-treated sample; dotted l i n e R2-treated sample which preadsorbed oxygen a t 77 K.
241
before ( s o l i d l i n e ) a n d a f t e r (dotted l i n e ) oxygen i n t e r a c t i o n f o r a C3 R2-treated sample. The band a t 2098 cm-l i s s h i f t e d t o 2116 cm-’, t h a t a t 2070 cm-l d i -1 sappears, a weak and sharper band grows a t 2140 cm . Microgravimetric measurements on R 2 and R3 - t r e a t e d samples show t h a t : i ) the r a t i o between the number of CO molecules i r r e v e r s i b l y adsorbed a t 77 K and the t o t a l number of copper atoms i s 0.22
+
0.24; i i ) the r a t i o between the number o f
oxygen atoms i r r e v e r s i b l y adsorbed a t 77 K a n d the t o t a l number of copper atoms i s 0.14
f
0.16.
UV-VIS-NIR spectra of oxidized and reduced samples The experiments a r e performed only on C3 and C5 samples. I n the two cases s i milar r e s u l t s a r e obtained. Fig. 4 i l l u s t r a t e s typical spectra of a C3 sample. Solid curve r e f e r s t o f u l l y oxidized sample: d-d t r a n s i t i o n s of Cu(I1) i n t e t r a hedric coordination a t 6600 cm-l and the absorption edges of CuO and ZnO a t 11500 and 27200 cm-l a r e present.
I
I
25
I
20
I
15
IO
I
F i g . 4 . Diffuse r e f l e c t a n c e s p e c t r a of a C3 sample. Solid l i n e , f u l l y oxidized sample; dotted l i n e , a f t e r CO i n t e r a c t i o n a t 298 K; dashed l i n e , R3-treated sample. When CO i s adsorbed a t 298 K the d-d t r a n s i t i o n s a r e s l i g t h l y perturbed, and a -1 loss in reflectance extending from 11000 u p t o 27.200 cm appears (dotted curv e ) . Afte’r a R3 treatment (dashed curve) the sample loses r e f l e c t a n c e from 7.000
242
-1 u p t o 27.200 cm , d - d t r a n s i t i o n and C u O absorption edge disappear, while a ve-1 ry intense and broad absorption with maximum a t 18000 cm grows u p . This maxi-
mum i s in the region o f the e x c i t a t i o n o f the d-band electrons in the corduction band of metallic copper. These spectral changes can be associated with the f o r mation of small p a r t i c l e s a n d a defective reduced C u / Z n O phase ( r e f . 1). DISCUSSION
Fully oxidized samples IR experiments of CO adsorption a t 77 K and a t 298 K show t h a t a f a s t reduction process occurs. S o , f i r s t of a l l , i t i s impossible t o t e s t with CO a f u l l y oxidized surface, n o t even a t 77 K . The d i f f e r e n t components appearing i n t h e I R spectra and the associated surface species a r e reported in Table 1 . TABLE 1
-1
species
f requencies/cm
Z n ( I1)CO
21 78-93
Cu(I1)CO
2172
r e v e r s i b l e a t 77 K
2152
r e v e r s i b l e a t 77 K
Cu( 1 ) C O
21 30
r e v e r s i b l e a t 298-373 K
Cu( 1)CO
21 10
r e v e r s i b l e a t 298 K
Cu(0)CO
2098
i r r e v e r s i b l e a t 77 K, r e v e r s i b l e a t 298 K
OH--
-CO
oAo k
k
0'
09"O
r e v e r s i b l e a t 77 K, blues h i f t a t decreasing coverage
1575, 1326 1015, 850
abundant a t 77 K, grow a t 298 K , unstable in vacuo a t 423-473 K
1520, 1330 1047, 856
scarse a t 77 K, abundant a t 298 K, grow u p a t 473 K , unstable in vacuo a t 573 K
M
h k
features
243
The assignments
have been made on t h e b a s i s of a wide l i t e r a t u r e . I n a number
4, 5 ) i t has been observed t h a t i n t h e Cu(1I)CO complexes t h e -1 . CO s t r e t c h i n g v b r a t i o n s a r e s i t u a t e d w i t h i n t h e i n t e r v a l 2210-2170 cm , i n t h e
o f studies ( r e f
Cu(1)CO complexes w i t h i n t h e i n t e r v a l 2160-2120 cm-l, w h i l e t h e s t r e t c h i n g v i b r a t i o n s o f Cu(o)CO complexes have been founded below 2110 cm-’.
Moreover M(I1)CO
complexes a r e g e n e r a l l y c h a r a c t e r i z e d by low s t a b i l i t y , owing t o t h e n a t u r e o f t h e i o n carbon i n t e r a c t i o n , e s s e n t i a l l y u i n t h e h i g h l y c h a r g e d i o n s , and t o t h e weak e l e c t r o n donor p r o p e r t i e s o f t h e CO m o l e c u l e . M(1)CO and Mjo)CO
complexes a r e c h a r a c t e r i z e d by a h i g h e r s t a b i l i t y , owing t o
t h e a b i l i t y o f low v a l e n c e i o n s and metal atoms t o f o r m
71
bond w i t h CO m o l e c u l e .
A s y n e r g i c u-T e f f e c t , p a r t i c u l a r l y i m p o r t a n t f o r M(1)CO complexes, can someti-
mes make them more s t a b l e t h a n M(o)CO ones. T h i s has been e s t a b l i s h e d f o r copper i n CuO/A1 0 ( r e f . 5 ) . On t h e b a s i s o f t h e s e c o n s i d e r a t i o n s we can comment t h e 2 3 assignment o f T a b l e 1 . The band a t 2152 cm-l i s e a s i l y assigned t o CO p h y s i s o r b e d on s u r f a c e h y d r o x y l s (ref. 3). The bands a t h i g h e r f r e q u e n c i e s , r e a d i l y decomposed i n vacuo a t 77 K, a r e a s s i gned t o CO complexes on d i v a l e n t c a t i o n s . The b e h a v i o u r o f t h e 2178 cm-l band ( p o s i t i o n , b l u e - s h i f t w i t h d e c r e a s i n g coverage, t i m e o f e v a c u a t i o n ) a l l o w s us t o a s s i g n i t t o CO s p e c i e s adsorbed on Zn i o n s of stepped (1071) f a c e s o f ZnO phase ( r e f . 3 ) . On t h e c o n t r a r y i t i s d i f f i c u l t t o s t a t e t h e c a t i o n f o r t h e complex abs o r b i n g a t 2172 ctn-l. I n f a c t on p u r e ZnO a band a t 2169 cm-l i s always p r e s e n t when CO i s adsorbed a t 77 K, w i t h t h e same s t a b i l i t y o f t h e 2172 cm-l one, a s s i gned t o CO on Zn i o n s o f p r i s m a t i c (1070) f a c e s . The 2172 cm-’
band can f i n a l l y
be assigned t o a Cu(1I)CO s p e c i e s o n l y on t h e b a s i s o f t h e r e s u l t s o b t a i n e d on
H -reduced samples. I n f a c t i t i s s t i l l p r e s e n t a f t e r R1 t r e a t m e n t s , even i f de2 creased i n i n t e n s i t y , and d i s a p p e a r s by complete r e d u c t i o n a f e e r R2 o r R3 t r e a t ments. The presence of t h e 2172 cm-l band on R 2 - t r e a t e d samples i n d i c a t e s t h e presence of copper i o n s
w i t h a s t r o n g r e s i s t a n c e t o t h e r e d u c t i o n ; i t s h o u l d be
t h e c h a r a c t e r i s t i c o f c u p r i c i o n s w i t h some oxygens o f t h e ZnO m a t r i x i n t h e i r c o o r d i n a t i o n sphere,
o r i s o l a t e d i n ZnO m a t r i x .
The component a t 2098 cm-l i s assigned t o Cu(o)CO complexes, b o t h f o r i t s f r e quency and s t a b i l i t y . The 2130 and 2110 cm-l bands, t h e most s t a b l e ones, a r e assigned t o two d i f f e r e n t Cu( 1)CO species, t h e f i r s t one c h a r a c t e r i s t i c o f l e s s reduced areag, where
244
t h e e l e c t r o n a v a i l a b i l i t y i s poor, t h e second one c h a r a c t e r i s t i c o f more reduced areas, where e l e c t r o n a v a i l a b i l i t y i s l a r g e . We can i n t e r p r e t t h e r e d u c t i o n process i r CO as f o l l o w s . There a r e some zones o f CuO phase so r e d u c i b l e where, even i n CO a t 77 K, copper i s e a s i l y reduced t o Cu(o) w h i l e i n o t h e r zones t h e r e d u c t i o n goes o n l y t o Cu(1). The r a t i o between t h e Cu(o) and t h e two Cu(1) complexes i s such t h a t an u n s t r u c t u r a t e d a b s o r p t i o n -1 component by ew i t h maximum a t 2115 cm-’ appears. The i n c r e a s e o f t h e 2130 cm v a c u a t i o n a t 298 K i s i n t e r p r e t e d as a p r o d u c t o f r e d u c t i o n o f C u ( I 1 ) t o Cu(1) i n t h o s e s u r f a c e areas more r e s i s t a n t t o t h e r e d u c t i o n , t h e CO b e i n g p r o v i d e d by Cu(o)CO species which decompose a t 298 K . The process o f r e d u c t i o n can c o n t i n u e when t h e temperature i s i n c r e a s e d . The r e a d s o r p t i o n o f CO a t 7 7 K r e v e a l s i n f a c t a d i f f e r e n t i n t e n s i t y r a t i o o f t h e t h r e e components, Cu(1) species c h a r a c t e r i s t i c o f more reduced s i t u a t i o n s and Cu(o) species a r e favoured. I n t h i s a s i t u a t i o n t h e number of CO molecules i r r e v e r s i b l y taken a t 77 K i s such t h a t 32-34% o f copper appears t o be exposed, i f we assume a r a t i o 1 : l Cu t o CO. Keeping i n mind t h a t t h e C u ( I 1 ) complexes, u n s t a b l e i n vacuo, a r e n o t counted, t h i s percent a g e must be c o n s i d e r e d a l o w e r l i m i t f o r t h e measure o f t h e copper e x p o s i t i o n . T h i s v a l u e shows a h i g h degree o f d i s p e r s i o n o f CuO phase, a c c o r d i n g w i t h t h e
XRD d a t a .
As f o r t h e bands i n t h e 1600-1800 cm-l r e g i o n , d i f f e r e n t experiments show t h a t t h e 1575, 1326, 1015, 850 cm-l ones a r e w e l l c o r r e l a t e d , and s i m i l a r l y t h e 1520, 1330, 1047, 856 cm-’ ones. The f i r s t c o r r e l a t e d band q u a r t e t i s assigned t o b i c o o r d i n a t e d carbonate species, t h e second one t o p o l y d e n t a t e species, b o t h f o r s p e c t r o s c o p i c reasons ( r e f . 6, 7) and on t h e b a s i s
o f t h e i r behaviour towards thermal t r e a t m e n t s . We t h i n k
t h a t t h e y a r e p r e f e r e n t i a l l y c o o r d i n a t e d t o Z n ( I 1 ) and Cu(11) i o n s , i n f a c t , a f t e r complete d e s o r p t i o n a t 588 Kand r e a d s o r p t i o n o f CO, M(I1)CO complexes grow up d e s p i t e o f t h e i n c r e a s e d r e d u c t i o n . The f a c t t h a t , i n c r e a s i n g copper l o a d i n g , t h e gaseous CO
p r e v a i l s on t h e carbo2 n a t e - l i k e adsorbed s p e c i e s - a s o x i d a t i o n p r o d u c t can be i n t e r p r e t e d as due t o t h e i n c r e a s e d masking, by t h e h i g h l y d i s p e r d e d CuO phase, o f t h e more b a s i c oxygen ions o f the
ZnO m a t r i x .
Reduced samples The 2103-2098 and 2070 cm-l bands on reduced samples show s t a b i l i t y and f r e -
24 5
q u e n c i e s t y p i c a l o f CO a d s o r b e d on m e t a l c o p p e r s u r f a c e s . I R r e f l e c t i o n e x p e r i ments on Cu m o n o c r y s t a l s i n d i c a t . e t h a t bands a t f r e q u e n c i e s h i g h e r t h a n 2094 cm
-I
a r e c h a r a c t e r i s t i c o f ( 1 1 0 ) o r h i g h e r i n d e x p l a n e s , w h i l e bands a t f r e q u e n -
a r e c h a r a c t e r i s t i c o f ( 1 1 1 ) and (1CO) p l a n e s . So on -1 t h e b a s i s o f t h e f r e q u e n c i e s we c o u l d s u g 9 e s t t o a s s i g n t h e 2103-2098 cm band c i e s l o w e r t h a n 2094 cm-’
t o CO c h e m i s o r b e d on ( 1 1 0 ) o r h i g h e r i n d e x m i c r o f a c e t s and t h e 2070 cm-l one t o CO a d s o r b e d on ( 1 1 1 ) o r ( 1 0 0 ) m i c r o f a c e t s .
Thi;
s h o u l d be a good a s s i g n m e n t f o r t h e 2103-2098 cm
-1
band, i n f a c t i t s shape
and h a l f w i d t h a r e t y p i c a l o f bands o f CO c h e m i s o r b e d on an e x t e n d e d o r d i n a t e s u r f a c e ( r e f . 8 ) . The 2070 cm-’ band on t h e c o n t r a r y shows a h a l f w i d t h t y p i c a l o f h i g h heterogeneous o r d i s o r d i n a t e s u r f a c e ( r e f . 8 ) . Some o t h e r i m p o r t a n t i n f o r m a t i o n s can be d e r i v e d b y t h e s t u d y o f CO a d s o r b e d on R2 o r R3 r e d u c e d samples t h a t p r e v i o u s l y have c h e m i s o r b e d oxygen a t 77 K . T h i s i n t e r a c t i o n r e v e a l s a d i f f e r e n t behaviour o f t h e two metal s i t e s i n v o l v e d .
The
2098 cm-l s h i f t s t o 2116 c m - ’ , i t s
h a l f width a n d i n t e n s i t y being almost unaffec-
t e d ; t h e b r o a d band a t 2070 cm-’
i s m i s s i n g and a new band a t
%
2140 cm-l appe-
a r < , d i f f e r e n t i n shape and h a l f w i d t h , showing an i n t e g r a t e d i n t e n s i t y c o r r e l a t e d t o t h a t o f t h e 2070 cm-’ The s h i f t o f t h e 2098 cm-’
band.
band i s t h e t y p i c a l one shown b y CO c h e m i s o r b e d on
( 1 1 0 ) f a c e o f c o p p e r m o n o c r y s t a l t h a t have p r e a d s o r b e d oxygen a t 80 K ( r e f . 9 ) . The v e r y l a r g e s h i f t ( 7 0 cm-’)
o f t h e 2070 cm-’
band and t h e c o m p l e t e change i n
i t s shape i n d i c a t e s a m a j o r p e r t u r b a t i o n . I t i s w e l l known t h a t t h e m o s t u n c o o r d i n a t e d atoms a r e t h e m o s t r e a c t i v e t o w a r d s t h e o x y g e n , sowe a s s i g n t h e band t o CO a d s o r b e d on c o p p e r stoms h i g l y u n c o o r d i n a t e d such t h o s e o f m e t a l p a r t i c l e b o r d e r l i n e s . The 2140 cm-’
band i s a s s i g n e d t o CO a d s o r b e d on t h e same o x i d i z e d atoms.
M i c r o g r a v i m e t r i c e x p e r i m e n t s show t h a t a l a r g e f r a c t i o n o f c o p p e r atoms i s expos e d on t h e s u r f a c e . I f we assume no b u l k d i f f u s i o n o f oxygen a t 77 K and a r a t i o 2 : l between s u r f a c e m e t a l atoms and oxygen c h e m i s o r b e d atoms, t h e n a b o u t 28-32% o f c o p p e r atoms w i l l b e a t t h e s u r f a c e . We f u r t h e r n o t e t h a t a t a l l
r e d u c t i o n s t a g e s , CO a d s o r p t i o n a t 77 K shows t h e
p r e s e n c e o n t h e s u r f a c e o f complexes a s s i g n e d t o CO c h e m i s o r b e d on s t e p p e d f a c e s (1011) o f ZnO. No e v i d e n c e s a r e f o u n d e d on t h e c o n t r a r y o f complexes a s s i g n e d t o CO c h e m i s o r b e d o n p r i s m a t i c (1070) f a c e s of ZnO. T h i s s u g g e s t s t h a t Zn i o n s o f
t h e l a t t e r f a c e s a r e c o m p l e t e l y h i d e n o r s u b s t i t u t e d b y c o p p e r atoms and t h a t c o p p e r m e t a l p a r t i c l e s g r o w p r e f e r e n t i a l l y on t h i s f a c e . A s i m i l a r r e s u l t was ob-
246
t a i n e d , w i t h d i f f e r e n t t e c h n i q u e s , by K l i e r e t a l . ( r e f .
1 ) f o r a similar Cu/
ZnO phase.
REFERENCES 1 K . K l i e r , Adv. C a t a l . , 31 (1982) 243-313. 2 C . P e t r i n i , F. Montino, A . Bossi and F . G a r b a s s i , S c i e n t i f i c b a s i s f o r the P r e p a r a t i o n of Heterogeneous c a t a l y s t s , Paper H5. 1-10, Louvain-La-Neuve ( 1982). 3 G . G h i o t t i , F. Boccuzzi, and R. S c a l a , J . C a t a l . , i n p r e s s . 4 Yu. A . Lokhov, V . I . Z a i k o v s k i i and A . A . Solomennikov, Kin. K a t a l . , 2? (1 982) 348-355. 5 N . Sheppard and T . T . Nguyen, i n R . J . H . Clark and R . E . H e s t e r ( E d s . ) , Advances i n I n f r a r e d and Raman S p e c t r o s c o p y , Vol. 5 , Heyden, London, 1978, C h . 2 , p . 67. 6 G . Busca and V . L o r e n z e l l i , Mater. Chem., 7 (1982) 89-126. 7 J . S a u s s e y , J . 6. L a v a l l e y and C . Bovet, J . Chem. S O C . , Faraday T r a n s . 1 , 78 (1982) 1457-1463. 8 F. M . Hoffmann, S u r f a c e S c i . Rep., 3 (1983) 107-192. 9 P . H o l l i n s and J . P r i t c h a r d , S u r f a c e S c i . , 134 (1983) 91-108.
Acknowledgments - W e thank the I t a l i a n C o n s i g l i o Nazionale d e l l e R i c e r c h e "Prog e t t o f i n a l i z z a t o : Chimica f i n e e s e c o n d a r i a " f o r f i n a n c i a l s u p p o r t .
23 5
SURFACE C H A R A C T E R I Z A T I O N OF Cu/ZnO CATALYSTS: I R , V I S , U V STUDY OF CO CHEMISORPTION
G. G H I O T T I , F. BOCCUZZI and A. C H I O R I N O I s t i t u t o d i Chimica F i s i c a , C.so
M. D ’ A z e g l i o 48, 10125 T o r i n o ( I t a l y )
ABSTRACT
The s t a t e o f copper i n Cu/ZnO c a t a l y s t s a t d i f f e r e n t r e d u c t i o n stages were s t u d i e d , m a i n l y by c h e m i s o r p t i o n o f CO a t 77 K. The s p e c t r o s c o p i c d a t a o f f u l l y o x i d i z e d samples i n d i c a t e t h a t a l a r g e p a r t o f copper i o n s can be r e a d i l y reduced t o Cu(o) o r Cu(1) by CO a t 77 K . Two d i f f e r e n t Cu(1)CO complexes a r e p r e sent, a b s o r b i n g a t 2130 and 2110 cm-’ r e s p e c t i v e l y . C u p r i c i o n s s t r o n g l y r e s i s t a n t t o t h e r e d u c t i o n g i v e r i s e t o Cu(1I)CO complex a b s o r b i n g a t 2172 cni’unstab l e i n vacuum. C a r b o n a t e - l i k e species appear as o x i d a t i o n p r o d u c t s , m a i n l y adsorbed on Z n ( I 1 ) and C u ( I 1 ) i o n s . On f u l l y reduced samples two chemisorbed spec i e s a r e p r e s e n t . One a b s o r b i n g a t 2098 cm-l, i s assigned t o CO chemisorbed on h i g h i n d e x m i c r o f a c e t s o f m e t a l p a r t i c l e s . The o t h e r , a b s o r b i n g a t 2070 cm-’, i s assigned t o CO chemisorbed on edges and c o r n e r s o f metal p a r t i c l e s .
INTROOUCTION A f t e r t h e i n t r o d u c t i o n o f Cu/ZnO/Al 0 and Cu/ZnO/Cr203 c a t a l y s t s f o r t h e low 2 3 p r e s s u r e methanol s y n t h e s i s and low temperature w a t e r s h i f t r e a c t i o n , many works ( r e f . 1) have e s t a b l i s h e d t h a t t h e h i g h a c t i v i t y r e s i d e s i n t h e b i n a r y copperz i n c o x i d e system. The aim o f t h i s work i s t o b r i n g a c o n t r i b u t i o n t o t h e knowl e d g e o f t h e f a c t o r s which c o n t r i b u t e t o t h e a c t i v i t y o f t h i s b i n a r y system, s t a r t i n g f r o m a p r e c u r s o r c o n s t i t u t e d by a s i n g l e phase Cu Zn mixed b a s i c carbona t e .
EXPERIMENTAL The c a t a l y s t p r e p a r a t i o n The s i n g l e phase p r e c u r s o r corresponds t o a s o l i d s o l u t i o n o f copper i n t h e h y d r o z i n c i t e s t r u c t u r e (Cu, Zn) ( C O ) (OH) p r e p a r e d and c h a r a c t e r i z e d by P e t r i 5 3 2 6 n i e t a l . ( r e f . 2 ) . Samples w i t h d i f f e r e n t Cu c o n t e n t were prepared. They a r e id e n t i f i e d by symbols Cx, where x r e p r e s e n t s t h e Cu/(CutZn) a t o m i c percentage. S i n g l e phase samples were c a l c i n e d a t 588 K and c h a r a c t e r i z e d by P e t r i n i e t a l .
236
( r e f . 2) w i t h XRO p a t t e r n s . C a l c i n e d samples examined, C3, C5 and C10
showed
t h e XRO p a t t e r n s o f ZnO and CuO w i t h i n t e n s i t y p r o p o r t i o n a l t o t h e c a t i o n concent r a t i o n s . C r i s t a l l i t e s i z e s determined by t h e S h e r r e r f o r m u l a were 15-20 nm and 6-8 nm f o r ZnO and CuO r e s p e c t i v e l y . C a l c i n e d samples powdered o r compressed i n p e l l e t s were s u b m i t t e d t o a l t e r n a t e t r e a t m e n t s i n vacuo and i n p u r e oxygen s l o w l y i n c r e a s i n g temperature f r o m 298 t o 588
K. We name samples
so t r e a t e d f u l l y o x i d i z e d samples.
2 The s p e c i f i c s u r f a c e areas o f f u l l y o x i d i z e d samples a r e 50, 40 and 40 m / g r f o r
C3, C5 and C10 r e s p e c t i v e l y . Reduced samples were prepared w i t h t h e f o l l o w i n g processes: R1) r e d u c t i o n i n a i n N m i x t u r e i n c r e a s i n g temperature up t o 453 K; R2) r e d u c t i o n i n 0 , 5 2 2 3% H i n N m i x t u r e s i n c r e a s i n g temperature up t o 503 K; R3) r e d u c t i o n i n 0.5-3% 2 2 H i n N m i x t u r e s and i n p u r e H i n c r e a s i n g temperature up t o 503 K . 2 2 2 The H consumption by TPR ( r e f . 2) i n d i c a t e s t h e complete r e d u c t i o n o f c u p r i c 2 o x i d e t o m e t a l l i c copper b o t h i n R2 and R3 processes.
0.5% H
Copper p a r t i c l e s were n o t d e t e c t a b l e by XRO o r TEM a n a l y s i s .
Experimental t e c h n i q u e s The I R s p e c t r a were r e c o r d e d w i t h a Perkin-Elmer 580 B i n t e r f a c e d w i t h
a
3600 Data S t a t i o n . The p e l l e t was mounted i n a s u i t a b l e I R c e l l which a l l o w e d b o t h h e a t i n g and cool i n g (down t o 77 surements a t 77 p r o x i m a t e l y 7-10
K) i n s i t u under vacuum o r c o n t r o l l e d atmosphere. D u r i n g I R meaK some h e a t i n g occurs a t t h e c e n t e r of t h e p e l l e t , v a l u a b l e apK. The U V - V I S - N I R d i f f u s e r e f l e c t a n c e
s p e c t r a were r e c o r d e d
by u s i n g a V a r i a n 2390 spectrophotometer equipped w i t h a d i f f u s e r e f l e c t a n c e sphere and i n t e r f a c e d w i t h an Apple I 1 p l u s microcomputer. M i c r o g r a v i m e t r i c measurements were c a r r i e d o u t w i t h a S a r t o r i u s 4102 e l e c t r o b a l a n c e o p e r a t i n g i n vacuo and i n c o n t r o l l e d atmosphere.
RESULTS I R s p e c t r a and m i c r o g r a v i m e t r i c measurements o f CO a d s o r p t i o n A d s o r p t i o n a t 77 K on f u l l y o x i d i z e d C3 samples. When CO i s adsorbed a t 77 K s p e c t r a l changes occur i n t h e h y d r o x y l s t r e t c h i n g r e g i o n (3800-3000 cm-I), d r o x y l bending and s u r f a c e mode ( r e f . 3 ) r e g i o n (800-6C0 cm-’), r e g i o n (1600-800 cm-l),
hy-
carbonate mode
and c a r b o n i l i c s p e c i e s r e g i o n (2000-2300 cm-’).
A typi-
231
cal experiment i s i l l u s t r a t e d in Fig. 1 , where only the 2300-2000 crn-' a n d the 1600-800 cm-l regions a r e reported.
O.D.
f
It\ t
', ,2115
1.o.
1575,
1330
0.5
2100
2ibo
1660
Fig. 1 . CO adsorption a t 77 K on a C3 f u l l y oxidized sample ( s o l i d l i n e ) and successive desorption a t increasing temperatures ( o t h e r symbols, see in t h e t e x t ) . Solid curve r e f e r s t o bands immediately appearing when the sample i s contacted with 25 Torr of CO. I n the 2300-2000 cm-l range a broad (FWHM = 80 cm-') and intense absorption appears with maximum a t 2115 cm
-1
and two sharper components a t
2152 and 2178 cm-l. I n t h e 1600-800 cm-l range four complexabsorptions a r e pre-1 sent: two more important a t 1570-1520 cm-l and a t 1340-1320 cm , and two l e s s important a t 1047-1015 cm-l and a t 856-850 cm-l ( t h i s l a s t not reported in figure). The band a t 2152 cm-l disappears f i r s t when t h e gas phase i s evacuated, simultaneouslythe surface hydroxyl s t r e t c h i n g and bending modes a r e completely restored in t h e i r original positions and half widths. The shoulder a t 2178 cm-l gradually
238
decreases showing a b l u e - s h i f t and disappears a f t e r 15 min of evacuation (dotted curve). Other
b a n d s a r e unaffected by desorption a t 77 K .
Further modifications a r e v i s i b l e only by desorption a t higher temperature. Dashed curve in Fig. 1 represents the evolution of the spectrum a f t e r 1 5 min of evacuation a t 298 K . The broad absorption a t 2115 cm-l i s now sharper, increased in i n t e n s i t y and s h i f t e d t o higher frequency ( a component a t 2098 cm-’ has been eroded, one a t 2130 cm-l i n c r e a s e d ) . A t the same time the complex absorptions in the 1600-800 cm-’ range grow u p . A prolonged outgassing a t 298 K f i n a l l y decreases the 2130 cm-l band while some increments in the carbonate modes s t i l l occur.
A t 373 K the 2130 cm-’ band disappears (black c i r c l e curve). Other bands remarkably decrease only by desorption a t 573 K (dot-dashed curve). After t h i s complete desorption the sample i s cooled down again t o 77 K and CO i s readmitted. I n Fig. 2 t h e recorded spectrum ( s o l i d l i n e ) i s compared with t h a t obtained a f t e r the f i r s t i n t e r a c t i o n (dashed l i n e ) .
F i g . 2. Comparison between spectra o f CO adsorbed a t 77 K on: a C3 f u l l y oxidized sample (dotted l i n e ) ; and the same C3 sample which a f t e r the f i r s t CO adsorption has been completely cleaned by evacuation a t 573 K ( s o l i d l i n e ) .
239
The most e v i d e n t d i f f e r e n c e s a r e : i)no bands appear i n t h e 1600-800 cm-l r e g i o n -1 ( n o t r e p o r t e d i n f i g u r e ) ; i i ) i n t h e 2300-2000 cm range t h e i n t e n s e and broad a b s o r p t i o n i s now more s t r u c t u r a t e d , t h r e e components a r e e v i d e n t , one a t 2098 -1 cm , another, t h e most i n t e n s e one, a t 2110 cm-l, and f i n a l l y one v e r y weak a t -1 2130 cm A l l t h e s e components a r e i r r e v e r s i b l e a t 77 K. A t h i g h e r f r e q u e n c i e s
.
s h a r p e r components a r e p r e s e n t a t 2152, ( n o t e v i d e n t a t t h e r e p o r t e d coverage, = 0.5 T o r r ) , 2172 and 2178 cm-’, a l l r e v e r s i b l e f o r e v a c u a t i o n a t 77 K i n pco 2178 cm-l. The l a s t one shows a b l u e s h i f t t i l l 2193 t h e sequence 2152 > 2172 -1 cm
.
M i c r o g r a v i m e t r i c measurements performed on samples s o t r e a t e d show t h a t t h e r a t i o between t h e number o f CO molecules i r r e v e r s i b l y adsorbed a t 77 K and t h e t o t a l numbers o f copper atoms c o n t a i n e d i n t h e sample i s 0.32-0.34. Samples c o m p l e t e l y c l e a n e d by e v a c u a t i o n a t 573 K and c o o l e d t o 298 K l o s e 0.450.5% o f t h e i r w e i g h t . I f we i n t e r p r e t t h e w e i g h t l o s s as c o m p l e t e l y due t o oxygen l o s t f o r r e d u c t i o n , t h e r a t i o between t h e number o f l o s t oxygen atoms and readsorbed a t 2 i s such t h a t t h e r a t i o between t h e number o f taken oxygen atoms and t h e t o -
t h e t o t a l number o f copper atoms i s 0.11:0.13. 298
K
The w e i g h t o f 0
t a l number o f copper atoms i s 0.11. A d s o r p t i o n o f CO a t 77 K on t h e s e r e o x i d i z e d samples g i v e s r i s e t o s p e c t r a and g r a v i m e t r i c d a t a p r a c t i c a l l y c o i n c i d e n t w i t h those observed a f t e r f i r s t CO c o n t a c t on f u l l y o x i d i z e d samples. A d s o r p t i o n a t 298
K on f u l l y o x i d i z e d C3 samples. When CO (10 T o r r ) i s adsor-
bed a t 298 K on f u l l y o x i d i z e d samples, s p e c t r a l changes a r e v e r y s i m i l a r t o those observed f o r t h e i n t e r a c t i o n a t 77 K, b u t no m o d i f i c a t i o n s a r e now v i s i b l e i n t h e h y d r o x y l modes. I n t h e 2300-2000 cm-l range t h e broad a b s o r p t i o n w i t h maxi-
mum a t 2110 cm-’ appears w h i l e t h e h i g h e r f r e q u e n c y bands a r e m i s s i n g . I n t h e 1600-1800 cm-l range t h e f o u r complex a b s o r p t i o n s a r e p r e s e n t . The i n t e n s i t i e s o f a l l bands a r e now h i g h e r t h a n those observed f o r t h e i n t e r a c t i o n a t 77 K . A d s o r p t i o n a t 77 K and 298 K on f u l l y o x i d i z e d C5 and C10 samples. The e x p e r i ments p r e v i o u s l y d e s c r i b e d f o r C3 samples, p e r f o r m e d on C5 and C10 samples l e a d t o s i m i l a r r e s u l t s . The most e v i d e n t d i f f e r e n c e s a r e : i)t h e r a t i o between t h e i n t e g r a t e d i n t e n s i t y of t h e carbonate
-
l i k e s p e c i e s modes and t h a t o f t h e carbo-
n i l i c species decreases w i t h i n c r e a s i n g copper c o n t e n t ; i i ) spectrum o f gaseous C02 i s now v i s i b l e when i n t e r a c t i o n i s performed a t 298 K; i i i ) t h e bands o f t h e c a r b o n i l i c species show now a h i g h e r r e v e r s i b i l i t y t o t h e e v a c u a t i o n a t 298 K.
240
Adsorption a t 77 K on reduced samples. When CO i s adsorbed a t 77 K on samples (C3, C5, C10) reduced with the R1 treatment, no bands in the 1600-800 cm-’ range appear. Spectral changes occur i n a l l the other examined regions. I n the 2300-1 2000 cm range the broader and intense absorption a t lower frequency has been s u b s t i t u t e d by two new bands: one sharp (FWHM t y a t 2103 cm-’ and one broader (FWHM
=
40
=
ern-')
10 cm-’) a n d o f medium i n t e n s i at
.?i
2070 cm-’ .They a r e s t a b l e
under vacuum a t 77 K, unstable a t 298 K. Bands a t 2152, 2172 a n d 2178 cm-’ a r e s t i l l present, with t h e same f e a t u r e s described f o r oxidized samples. When CO i s adsorbed a t 77 K on R2 o r R3-treated samples the two bands, now a t -1 band 2098 and x 2070 cm-l, a r e present with increased i n t e n s i t y . The 2 1 7 2 cm i s completely missing, the 2178 cm-l one i s s t i l l present. Fig. 3 a shows a comparison between C O adsorbed on a C3 R1-treated sample (dotted l i n e ) a n d on a C3 R3-treated sample ( s o l i d l i n e ) a t a CO equilibrium pressure o f 0 . 5 Torr. Clean reduced samples a r e contacted with oxygen a t 77 K, evacuated, then CO i s adsorbed a t the same temperature. Fig. 3 b shows a comparison between CO spectra
O.D.
a
b 21.72
1.0-
,”\ f
2Q90
2098
0.5-
I
2200
I
2100
I
2200
2 l b
31cm“
Fig. 3. CO adsorption a t 77 K on C3 reduced samples. a ) Dotted l i n e , R1-treated sample; s o l i d l i n e , RP-treated sample. b ) Solid l i n e , R2-treated sample; dotted l i n e R2-treated sample which preadsorbed oxygen a t 77 K.
241
before ( s o l i d l i n e ) a n d a f t e r (dotted l i n e ) oxygen i n t e r a c t i o n f o r a C3 R2-treated sample. The band a t 2098 cm-l i s s h i f t e d t o 2116 cm-’, t h a t a t 2070 cm-l d i -1 sappears, a weak and sharper band grows a t 2140 cm . Microgravimetric measurements on R 2 and R3 - t r e a t e d samples show t h a t : i ) the r a t i o between the number of CO molecules i r r e v e r s i b l y adsorbed a t 77 K and the t o t a l number of copper atoms i s 0.22
+
0.24; i i ) the r a t i o between the number o f
oxygen atoms i r r e v e r s i b l y adsorbed a t 77 K a n d the t o t a l number of copper atoms i s 0.14
f
0.16.
UV-VIS-NIR spectra of oxidized and reduced samples The experiments a r e performed only on C3 and C5 samples. I n the two cases s i milar r e s u l t s a r e obtained. Fig. 4 i l l u s t r a t e s typical spectra of a C3 sample. Solid curve r e f e r s t o f u l l y oxidized sample: d-d t r a n s i t i o n s of Cu(I1) i n t e t r a hedric coordination a t 6600 cm-l and the absorption edges of CuO and ZnO a t 11500 and 27200 cm-l a r e present.
I
I
25
I
20
I
15
IO
I
F i g . 4 . Diffuse r e f l e c t a n c e s p e c t r a of a C3 sample. Solid l i n e , f u l l y oxidized sample; dotted l i n e , a f t e r CO i n t e r a c t i o n a t 298 K; dashed l i n e , R3-treated sample. When CO i s adsorbed a t 298 K the d-d t r a n s i t i o n s a r e s l i g t h l y perturbed, and a -1 loss in reflectance extending from 11000 u p t o 27.200 cm appears (dotted curv e ) . Afte’r a R3 treatment (dashed curve) the sample loses r e f l e c t a n c e from 7.000
242
-1 u p t o 27.200 cm , d - d t r a n s i t i o n and C u O absorption edge disappear, while a ve-1 ry intense and broad absorption with maximum a t 18000 cm grows u p . This maxi-
mum i s in the region o f the e x c i t a t i o n o f the d-band electrons in the corduction band of metallic copper. These spectral changes can be associated with the f o r mation of small p a r t i c l e s a n d a defective reduced C u / Z n O phase ( r e f . 1). DISCUSSION
Fully oxidized samples IR experiments of CO adsorption a t 77 K and a t 298 K show t h a t a f a s t reduction process occurs. S o , f i r s t of a l l , i t i s impossible t o t e s t with CO a f u l l y oxidized surface, n o t even a t 77 K . The d i f f e r e n t components appearing i n t h e I R spectra and the associated surface species a r e reported in Table 1 . TABLE 1
-1
species
f requencies/cm
Z n ( I1)CO
21 78-93
Cu(I1)CO
2172
r e v e r s i b l e a t 77 K
2152
r e v e r s i b l e a t 77 K
Cu( 1 ) C O
21 30
r e v e r s i b l e a t 298-373 K
Cu( 1)CO
21 10
r e v e r s i b l e a t 298 K
Cu(0)CO
2098
i r r e v e r s i b l e a t 77 K, r e v e r s i b l e a t 298 K
OH--
-CO
oAo k
k
0'
09"O
r e v e r s i b l e a t 77 K, blues h i f t a t decreasing coverage
1575, 1326 1015, 850
abundant a t 77 K, grow a t 298 K , unstable in vacuo a t 423-473 K
1520, 1330 1047, 856
scarse a t 77 K, abundant a t 298 K, grow u p a t 473 K , unstable in vacuo a t 573 K
M
h k
features
243
The assignments
have been made on t h e b a s i s of a wide l i t e r a t u r e . I n a number
4, 5 ) i t has been observed t h a t i n t h e Cu(1I)CO complexes t h e -1 . CO s t r e t c h i n g v b r a t i o n s a r e s i t u a t e d w i t h i n t h e i n t e r v a l 2210-2170 cm , i n t h e
o f studies ( r e f
Cu(1)CO complexes w i t h i n t h e i n t e r v a l 2160-2120 cm-l, w h i l e t h e s t r e t c h i n g v i b r a t i o n s o f Cu(o)CO complexes have been founded below 2110 cm-’.
Moreover M(I1)CO
complexes a r e g e n e r a l l y c h a r a c t e r i z e d by low s t a b i l i t y , owing t o t h e n a t u r e o f t h e i o n carbon i n t e r a c t i o n , e s s e n t i a l l y u i n t h e h i g h l y c h a r g e d i o n s , and t o t h e weak e l e c t r o n donor p r o p e r t i e s o f t h e CO m o l e c u l e . M(1)CO and Mjo)CO
complexes a r e c h a r a c t e r i z e d by a h i g h e r s t a b i l i t y , owing t o
t h e a b i l i t y o f low v a l e n c e i o n s and metal atoms t o f o r m
71
bond w i t h CO m o l e c u l e .
A s y n e r g i c u-T e f f e c t , p a r t i c u l a r l y i m p o r t a n t f o r M(1)CO complexes, can someti-
mes make them more s t a b l e t h a n M(o)CO ones. T h i s has been e s t a b l i s h e d f o r copper i n CuO/A1 0 ( r e f . 5 ) . On t h e b a s i s o f t h e s e c o n s i d e r a t i o n s we can comment t h e 2 3 assignment o f T a b l e 1 . The band a t 2152 cm-l i s e a s i l y assigned t o CO p h y s i s o r b e d on s u r f a c e h y d r o x y l s (ref. 3). The bands a t h i g h e r f r e q u e n c i e s , r e a d i l y decomposed i n vacuo a t 77 K, a r e a s s i gned t o CO complexes on d i v a l e n t c a t i o n s . The b e h a v i o u r o f t h e 2178 cm-l band ( p o s i t i o n , b l u e - s h i f t w i t h d e c r e a s i n g coverage, t i m e o f e v a c u a t i o n ) a l l o w s us t o a s s i g n i t t o CO s p e c i e s adsorbed on Zn i o n s of stepped (1071) f a c e s o f ZnO phase ( r e f . 3 ) . On t h e c o n t r a r y i t i s d i f f i c u l t t o s t a t e t h e c a t i o n f o r t h e complex abs o r b i n g a t 2172 ctn-l. I n f a c t on p u r e ZnO a band a t 2169 cm-l i s always p r e s e n t when CO i s adsorbed a t 77 K, w i t h t h e same s t a b i l i t y o f t h e 2172 cm-l one, a s s i gned t o CO on Zn i o n s o f p r i s m a t i c (1070) f a c e s . The 2172 cm-’
band can f i n a l l y
be assigned t o a Cu(1I)CO s p e c i e s o n l y on t h e b a s i s o f t h e r e s u l t s o b t a i n e d on
H -reduced samples. I n f a c t i t i s s t i l l p r e s e n t a f t e r R1 t r e a t m e n t s , even i f de2 creased i n i n t e n s i t y , and d i s a p p e a r s by complete r e d u c t i o n a f e e r R2 o r R3 t r e a t ments. The presence of t h e 2172 cm-l band on R 2 - t r e a t e d samples i n d i c a t e s t h e presence of copper i o n s
w i t h a s t r o n g r e s i s t a n c e t o t h e r e d u c t i o n ; i t s h o u l d be
t h e c h a r a c t e r i s t i c o f c u p r i c i o n s w i t h some oxygens o f t h e ZnO m a t r i x i n t h e i r c o o r d i n a t i o n sphere,
o r i s o l a t e d i n ZnO m a t r i x .
The component a t 2098 cm-l i s assigned t o Cu(o)CO complexes, b o t h f o r i t s f r e quency and s t a b i l i t y . The 2130 and 2110 cm-l bands, t h e most s t a b l e ones, a r e assigned t o two d i f f e r e n t Cu( 1)CO species, t h e f i r s t one c h a r a c t e r i s t i c o f l e s s reduced areag, where
244
t h e e l e c t r o n a v a i l a b i l i t y i s poor, t h e second one c h a r a c t e r i s t i c o f more reduced areas, where e l e c t r o n a v a i l a b i l i t y i s l a r g e . We can i n t e r p r e t t h e r e d u c t i o n process i r CO as f o l l o w s . There a r e some zones o f CuO phase so r e d u c i b l e where, even i n CO a t 77 K, copper i s e a s i l y reduced t o Cu(o) w h i l e i n o t h e r zones t h e r e d u c t i o n goes o n l y t o Cu(1). The r a t i o between t h e Cu(o) and t h e two Cu(1) complexes i s such t h a t an u n s t r u c t u r a t e d a b s o r p t i o n -1 component by ew i t h maximum a t 2115 cm-’ appears. The i n c r e a s e o f t h e 2130 cm v a c u a t i o n a t 298 K i s i n t e r p r e t e d as a p r o d u c t o f r e d u c t i o n o f C u ( I 1 ) t o Cu(1) i n t h o s e s u r f a c e areas more r e s i s t a n t t o t h e r e d u c t i o n , t h e CO b e i n g p r o v i d e d by Cu(o)CO species which decompose a t 298 K . The process o f r e d u c t i o n can c o n t i n u e when t h e temperature i s i n c r e a s e d . The r e a d s o r p t i o n o f CO a t 7 7 K r e v e a l s i n f a c t a d i f f e r e n t i n t e n s i t y r a t i o o f t h e t h r e e components, Cu(1) species c h a r a c t e r i s t i c o f more reduced s i t u a t i o n s and Cu(o) species a r e favoured. I n t h i s a s i t u a t i o n t h e number of CO molecules i r r e v e r s i b l y taken a t 77 K i s such t h a t 32-34% o f copper appears t o be exposed, i f we assume a r a t i o 1 : l Cu t o CO. Keeping i n mind t h a t t h e C u ( I 1 ) complexes, u n s t a b l e i n vacuo, a r e n o t counted, t h i s percent a g e must be c o n s i d e r e d a l o w e r l i m i t f o r t h e measure o f t h e copper e x p o s i t i o n . T h i s v a l u e shows a h i g h degree o f d i s p e r s i o n o f CuO phase, a c c o r d i n g w i t h t h e
XRD d a t a .
As f o r t h e bands i n t h e 1600-1800 cm-l r e g i o n , d i f f e r e n t experiments show t h a t t h e 1575, 1326, 1015, 850 cm-l ones a r e w e l l c o r r e l a t e d , and s i m i l a r l y t h e 1520, 1330, 1047, 856 cm-’ ones. The f i r s t c o r r e l a t e d band q u a r t e t i s assigned t o b i c o o r d i n a t e d carbonate species, t h e second one t o p o l y d e n t a t e species, b o t h f o r s p e c t r o s c o p i c reasons ( r e f . 6, 7) and on t h e b a s i s
o f t h e i r behaviour towards thermal t r e a t m e n t s . We t h i n k
t h a t t h e y a r e p r e f e r e n t i a l l y c o o r d i n a t e d t o Z n ( I 1 ) and Cu(11) i o n s , i n f a c t , a f t e r complete d e s o r p t i o n a t 588 Kand r e a d s o r p t i o n o f CO, M(I1)CO complexes grow up d e s p i t e o f t h e i n c r e a s e d r e d u c t i o n . The f a c t t h a t , i n c r e a s i n g copper l o a d i n g , t h e gaseous CO
p r e v a i l s on t h e carbo2 n a t e - l i k e adsorbed s p e c i e s - a s o x i d a t i o n p r o d u c t can be i n t e r p r e t e d as due t o t h e i n c r e a s e d masking, by t h e h i g h l y d i s p e r d e d CuO phase, o f t h e more b a s i c oxygen ions o f the
ZnO m a t r i x .
Reduced samples The 2103-2098 and 2070 cm-l bands on reduced samples show s t a b i l i t y and f r e -
24 5
q u e n c i e s t y p i c a l o f CO a d s o r b e d on m e t a l c o p p e r s u r f a c e s . I R r e f l e c t i o n e x p e r i ments on Cu m o n o c r y s t a l s i n d i c a t . e t h a t bands a t f r e q u e n c i e s h i g h e r t h a n 2094 cm
-I
a r e c h a r a c t e r i s t i c o f ( 1 1 0 ) o r h i g h e r i n d e x p l a n e s , w h i l e bands a t f r e q u e n -
a r e c h a r a c t e r i s t i c o f ( 1 1 1 ) and (1CO) p l a n e s . So on -1 t h e b a s i s o f t h e f r e q u e n c i e s we c o u l d s u g 9 e s t t o a s s i g n t h e 2103-2098 cm band c i e s l o w e r t h a n 2094 cm-’
t o CO c h e m i s o r b e d on ( 1 1 0 ) o r h i g h e r i n d e x m i c r o f a c e t s and t h e 2070 cm-l one t o CO a d s o r b e d on ( 1 1 1 ) o r ( 1 0 0 ) m i c r o f a c e t s .
Thi;
s h o u l d be a good a s s i g n m e n t f o r t h e 2103-2098 cm
-1
band, i n f a c t i t s shape
and h a l f w i d t h a r e t y p i c a l o f bands o f CO c h e m i s o r b e d on an e x t e n d e d o r d i n a t e s u r f a c e ( r e f . 8 ) . The 2070 cm-’ band on t h e c o n t r a r y shows a h a l f w i d t h t y p i c a l o f h i g h heterogeneous o r d i s o r d i n a t e s u r f a c e ( r e f . 8 ) . Some o t h e r i m p o r t a n t i n f o r m a t i o n s can be d e r i v e d b y t h e s t u d y o f CO a d s o r b e d on R2 o r R3 r e d u c e d samples t h a t p r e v i o u s l y have c h e m i s o r b e d oxygen a t 77 K . T h i s i n t e r a c t i o n r e v e a l s a d i f f e r e n t behaviour o f t h e two metal s i t e s i n v o l v e d .
The
2098 cm-l s h i f t s t o 2116 c m - ’ , i t s
h a l f width a n d i n t e n s i t y being almost unaffec-
t e d ; t h e b r o a d band a t 2070 cm-’
i s m i s s i n g and a new band a t
%
2140 cm-l appe-
a r < , d i f f e r e n t i n shape and h a l f w i d t h , showing an i n t e g r a t e d i n t e n s i t y c o r r e l a t e d t o t h a t o f t h e 2070 cm-’ The s h i f t o f t h e 2098 cm-’
band.
band i s t h e t y p i c a l one shown b y CO c h e m i s o r b e d on
( 1 1 0 ) f a c e o f c o p p e r m o n o c r y s t a l t h a t have p r e a d s o r b e d oxygen a t 80 K ( r e f . 9 ) . The v e r y l a r g e s h i f t ( 7 0 cm-’)
o f t h e 2070 cm-’
band and t h e c o m p l e t e change i n
i t s shape i n d i c a t e s a m a j o r p e r t u r b a t i o n . I t i s w e l l known t h a t t h e m o s t u n c o o r d i n a t e d atoms a r e t h e m o s t r e a c t i v e t o w a r d s t h e o x y g e n , sowe a s s i g n t h e band t o CO a d s o r b e d on c o p p e r stoms h i g l y u n c o o r d i n a t e d such t h o s e o f m e t a l p a r t i c l e b o r d e r l i n e s . The 2140 cm-’
band i s a s s i g n e d t o CO a d s o r b e d on t h e same o x i d i z e d atoms.
M i c r o g r a v i m e t r i c e x p e r i m e n t s show t h a t a l a r g e f r a c t i o n o f c o p p e r atoms i s expos e d on t h e s u r f a c e . I f we assume no b u l k d i f f u s i o n o f oxygen a t 77 K and a r a t i o 2 : l between s u r f a c e m e t a l atoms and oxygen c h e m i s o r b e d atoms, t h e n a b o u t 28-32% o f c o p p e r atoms w i l l b e a t t h e s u r f a c e . We f u r t h e r n o t e t h a t a t a l l
r e d u c t i o n s t a g e s , CO a d s o r p t i o n a t 77 K shows t h e
p r e s e n c e o n t h e s u r f a c e o f complexes a s s i g n e d t o CO c h e m i s o r b e d on s t e p p e d f a c e s (1011) o f ZnO. No e v i d e n c e s a r e f o u n d e d on t h e c o n t r a r y o f complexes a s s i g n e d t o CO c h e m i s o r b e d o n p r i s m a t i c (1070) f a c e s of ZnO. T h i s s u g g e s t s t h a t Zn i o n s o f
t h e l a t t e r f a c e s a r e c o m p l e t e l y h i d e n o r s u b s t i t u t e d b y c o p p e r atoms and t h a t c o p p e r m e t a l p a r t i c l e s g r o w p r e f e r e n t i a l l y on t h i s f a c e . A s i m i l a r r e s u l t was ob-
246
t a i n e d , w i t h d i f f e r e n t t e c h n i q u e s , by K l i e r e t a l . ( r e f .
1 ) f o r a similar Cu/
ZnO phase.
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Acknowledgments - W e thank the I t a l i a n C o n s i g l i o Nazionale d e l l e R i c e r c h e "Prog e t t o f i n a l i z z a t o : Chimica f i n e e s e c o n d a r i a " f o r f i n a n c i a l s u p p o r t .