Solid State Chemistry in Catalysis - American Chemical Society

12 Temperature-Programmed Decomposition of 2-Propanol on the Zinc-Polar, Nonpolar, and Oxygen-Polar Surfaces of Zinc Oxide K. LUI, S. AKHTER, and H. H. KUNG

Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch012

Chemical Engineering Department and Ipatieff Laboratory, Northwestern University, Evanston, IL 60201

The temperature programmed desorption and decomposition of 2-propanol, acetone, and propene were studied on the Zn-polar (0001), the stepped-nonpolar (5051), and the O-polar (0001) surfaces of ZnO.

The

desorption temperatures of all the species, except water, were the lowest on the O-polar surface, and the highest on the Zn-polar surface. Propene did not adsorb on the Zn-polar surface. Adsorbed acetone desorbed in two peaks from the nonpolar surface. On the other surfaces, adsorbed propene or adsorbed acetone desorbed in a single peak. 2-Propanol was decomposed into acetone, propene, H , and H O. The ratios of acetone to propene were the same independent of the surface and the 2-propanol coverage. The evolution of the products except Η O, was reaction limited. The decomposition activity was the highest on the Zn-polar face, and lowest on the O-polar face. 2

2

2

Studies i n recent years on the surface properties of t r a n s i t i o n metal oxides have demonstrated that the surface s t r u c t u r a l s t a b i l i t y , the surface e l e c t r o n i c structure, and the surface chemical r e a c t i v i t y depend on the c r y s t a l lographic orientation of the exposed surface and the presence of surface imperfection, such as steps and point defects ( 1 ) . ZnO i s one recent example. The natural surfaces of ZnO, which can be prepared i n a r e l a t i v e l y well-ordered state, include the Zn-polar (0001 ), the 0 - p o l a r (OOOT), and the nonpolar (1010) surfaces. (See Figure 1 f o r a schematic representation of these surfaces). These surfaces have been shown to possess d i f f e r e n t chemisorptive properties and reactivities. I t was shown that C 0 was desorbed from a nonpolar surface at about 120°C., but from a Zn-polar surface at 410°C ( 2 ) . The decomposition of methanol, formaldehyde, and formic a c i d a l s o d i f f e r e d (3,4)* In general, the temperature at which the decomposition products appeared was the highest on the Zn-polar 2

0097-6156/85/0279-0205$06.00/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, Robert K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

S O L I D STATE C H E M I S T R Y IN CATALYSIS


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12.

LUI ET AL.

Temperature-Programmed Decomposition of 2-Propanol

207

f a c e , i n t e r m e d i a t e on the n o n p o l a r f a c e , and the l o w e s t on the 0 p o l a r f a c e (5). The d e c o m p o s i t i o n a c t i v i t y , measured a s the f r a c t i o n o f adsorbed m o l e c u l e s decomposed, d e c r e a s e d i n the same o r d e r : Z n - p o l a r > n o n p o l a r > 0 - p o l a r . The r e a c t i o n p r o d u c t s i n methanol d e c o m p o s i t i o n a l s o d i f f e r e d . From t h e s e o b s e r v a t i o n s , i t was c o n c l u d e d t h a t on the Z n - p o l a r s u r f a c e , the r e a c t i o n o f methanol proceeded v i a b o t h o x i d a t i o n by the l a t t i c e , and dehydrogenation. The r a t i o o f o x i d a t i o n t o d e h y d r o g e n a t i o n v a r i e d a c c o r d i n g t o the c o n d i t i o n o f the s u r f a c e . On the n o n p o l a r and the 0 - p o l a r s u r f a c e s , o n l y t h e o x i d a t i o n pathway p a r t i c i p a t e s . These were i n t e r p r e t e d as an i n d i c a t i o n o f the presence o f b o t h m e t a l l i c and o x i d i c c h a r a c t e r on t h e Z n - p o l a r s u r f a c e , b u t o n l y the o x i d i c c h a r a c t e r on the o t h e r s u r f a c e s , w h i c h i s c o n s i s t e n t w i t h the surface atomic s t r u c t u r e s . S u c h a c r y s t a l p l a n e dependence o f the a d s o r p t i o n and r e a c t i o n p r o p e r t i e s o f ZnO has a l s o been shown u s i n g powder samples ( 6 ) . I n v i e w o f the i n t e r e s t i n g r e s u l t s o f methanol d e c o m p o s i t i o n , we p r o c e e d e d t o s t u d y t h e d e c o m p o s i t i o n o f 2 - p r o p a n o l . I t i s known t h a t 2 - p r o p a n o l decomposes v i a two competing pathways. 2-C^H 0H

—>

(CH ) C0 + H

2-C H 0H

—>

C3H5 + H 0

7

3

7

3

2

2

2

dehydrogenation dehydration

The d e h y d r o g e n a t i o n r e a c t i o n produces a c e t o n e and hydrogen, and i s dominant o v e r b a s i c o x i d e s ( 7 ) . The d e h y d r a t i o n r e a c t i o n produces propene and w a t e r , and i s dominant o v e r a c i d i c o x i d e s . I t w o u l d be i n t e r e s t i n g t o see i f the c o m p e t i t i o n between t h e s e two pathways depend on t h e exposed c r y s t a l p l a n e s o f ZnO. We r e p o r t here t h e r e s u l t s o f s u c h an i n v e s t i g a t i o n . 2 - P r o p a n o l was decomposed on ZnO s i n g l e c r y s t a l s u r f a c e s by t h e t e m p e r a t u r e programmed d e c o m p o s i t i o n technique. To a s s i s t the i n t e r p r e t a t i o n o f d a t a , the temperature programmed d e s o r p t i o n o f propene and a c e t o n e were a l s o s t u d i e d . Experimental E x p e r i m e n t s were c a r r i e d o u t i n a s t a n d a r d u l t r a h i g h vacuum system d e s c r i b e d before (2-4)» The system was e q u i p p e d w i t h a low energy e l e c t r o n d i f f r a c t i o n u n i t , an Auger e l e c t r o n s p e c t r o m e t e r , a s p u t t e r - i o n gun, and a UTI q u a d r u p o l e mass s p e c t r o m e t e r . The mass s p e c t r o m e t e r was c o m p u t e r - c o n t r o l l e d . I t had a s h i e l d i n f r o n t o f the i o n i z e r , t h a t had a h o l e o f 1 cm i n d i a m e t e r t o d i s c r i m i n a t e the mass s p e c t r o m e t e r s i g n a l due t o d i r e c t f l u x o f m o l e c u l e s from the background p r e s s u r e i n c r e a s e d u r i n g t e m p e r a t u r e programmed d e s o r p t i o n (TPD). TPD was performed by h e a t i n g the ZnO c r y s t a l a t 10 K s"'* r a d i a t i v e l y w i t h a t u n g s t e n f i l a m e n t b e h i n d t h e c r y s t a l . The ZnO c r y s t a l s were p u r c h a s e d from A t o m e r g i c C h e m i c a l s . The d i r e c t i o n o f t h e c - a x i s was d e t e r m i n e d by e t c h i n g ( 2 ) . The d e s i r e d s u r f a c e s were a b o u t 6 mm χ 6 mm. The edge and the back s i d e o f the c r y s t a l were c o v e r e d by e v a p o r a t e d g o l d . A d s o r p t i o n o f m o l e c u l e s was a c h i e v e d w i t h a d o s e r . Thus the e x p o s u r e s r e p o r t e d h e r e , w h i c h were c a l c u l a t e d w i t h t h e chamber p r e s s u r e i n d i c a t e d by an i o n gauge, were t e n t o one hundred t i m e s l e s s t h a n the a c t u a l exposure. P r e p a r a t i o n o f the (10T0), (5051), and (0001) s u r f a c e s has



208


been d e s c r i b e d b e f o r e (2). B r i e f l y , i t i n v o l v e d a l i g n m e n t o f the c r y s t a l by Laue x - r a y back s c a t t e r i n g , c u t t i n g o u t the d e s i r e d f a c e , m e c h a n i c a l p o l i s h i n g , and r e p e a t e d A r - i o n s p u t t e r i n g and a n n e a l i n g . The 0 - p o l a r (0001) f a c e was s i m i l a r l y p r e p a r e d . The a n n e a l i n g i n vacuum was performed a t 500°C. A 6 - f o l d "1 χ 1" LEED p a t t e r n was o b t a i n e d . S m a l l b u t d e t e c t a b l e (by Auger e l e c t r o n s p e c t r o s c o p y ) amounts o f S, C I , and Κ were p r e s e n t . The TPD r e s u l t s d i d n o t appear t o depend on the amounts o f t h e s e i m p u r i t i e s when the amount was s m a l l . As b e f o r e , t h r e e t y p e s o f c o n t r o l e x p e r i m e n t s were r o u t i n e l y performed t o h e l p d i s c r i m i n a t e the d e s i r e d TPD s i g n a l s from o t h e r d e s o r p t i o n s i g n a l s . The f i r s t type was a b l a n k r u n i n w h i c h the e n t i r e p r o c e d u r e o f TPD was c a r r i e d o u t e x c e p t t h a t no gases were i n t r o d u c e d onto the s u r f a c e . The second type was t o perform the TPD p r o c e d u r e b u t w i t h the sample f a c i n g away from the d o s e r d u r i n g adsorption. T h i s was used t o determine the amount o f a d s o r p t i o n on p l a c e s o t h e r t h a n the d e s i r e d sample s u r f a c e . The t h i r d t y p e was a l s o the same as an o r d i n a r y TPD e x p e r i m e n t e x c e p t t h a t d e s o r p t i o n was performed w i t h the s u r f a c e f a c i n g away from the mass s p e c t r o m e t e r . T h i s a l s o h e l p e d i d e n t i f y w h i c h d e s o r p t i o n was from the d e s i r e d s u r f a c e . These c o n t r o l e x p e r i m e n t s n o r m a l l y p r o v i d e d enough i n f o r m a t i o n f o r the i d e n t i f i c a t i o n o f the d e s i r e d d e s o r p t i o n s i g n a l s v e r s u s d e s o r p t i o n from background gas a d s o r p t i o n o r from o t h e r p l a c e s t h a n the d e s i r e d s u r f a c e . D e s o r p t i o n was m o n i t o r e d w i t h mass s p e c t r o s c o p y . The c r a c k i n g p a t t e r n s o f 2 - p r o p a n o l , a c e t o n e , and propene were i n d i v i d u a l l y d e t e r m i n e d (8). F o r q u a n t i t a t i v e a n a l y s i s , masses 4 5 , 4 3 , 4 1 , 18» and 2 were used f o r 2 - p r o p a n o l , a c e t o n e , propene, w a t e r , and hydrogen, r e s p e c t i v e l y , a f t e r c o r r e c t i o n f o r c r a c k i n g i n a s i m i l a r p r o c e d u r e as d e s c r i b e d (3,4)* The mass s p e c t r o m e t e r s e n s i t i v i t i e s w e r e d e t e r m i n e d t o be 5 . 2 6 , 7 . 8 8 , 3 . 0 7 , 4 - 7 4 , a n d 5 . 2 0 a m p / t o r r , and the pumping speeds were 9 . 5 , 13.1, 3 1 . 0 , 1.7, 36.9 L s e c " ' ' , r e s p e c t i v e l y f o r the f i v e s p e c i e s . These two l a t t e r q u a n t i t i e s were used t o c o n v e r t the mass s p e c t r o m e t e r r e a d i n g s i n t o m o l e c u l a r fluxes. Results As mentioned i n the p r e v i o u s s e c t i o n , c o n t r o l e x p e r i m e n t s were performed t o d i s c r i m i n a t e the d e s i r e d s i g n a l s from the o t h e r s i g n a l s . Thus, u n l e s s s p e c i f i e d , the r e s u l t s r e p o r t e d a r e the d e s i r e d s i g n a l s . Most o f the e x p e r i m e n t s were performed on the (0001), the (5051), and the (000T) s u r f a c e s , w h i c h w i l l a l s o be r e f e r r e d t o a s the Z n - p o l a r , n o n p o l a r , and the 0 - p o l a r s u r f a c e s , r e s p e c t i v e l y . The (5051) s u r f a c e was a c t u a l l y a s t e p p e d n o n p o l a r face. A few e x p e r i m e n t s were performed on the f l a t n o n p o l a r (1010) s u r f a c e a t h i g h 2 - p r o p o n o l exposures; the r e s u l t s were s i m i l a r t o t h e (5051) s u r f a c e . 2 - P r o p a n o l D e c o m p o s i t i o n . T y p i c a l TPD s p e c t r a f o r 2 - p r o p a n o l d e c o m p o s i t i o n on the (0001), (5051), and (0001) s u r f a c e s a r e shown i n F i g u r e s 2 a , b , and c, r e s p e c t i v e l y . Common t o a l l t h r e e s u r f a c e s was the e v o l u t i o n o f f i v e p r o d u c t s : undecomposed 2 p r o p a n o l , hydrogen, a c e t o n e , w a t e r , and propene. O t h e r s p e c i e s


LUI E T A L .

Temperature-Programmed Decomposition of 2-Propanol 1

1

1

1

ι


Τ

1

1

'—nb— —sfo— —s*—^ TEMPERATURE C F i g u r e 2 . T y p i c a l TPD s p e c t r a f o r 2 - p r o p a n o l d e c o m p o s i t i o n « d i f f e r e n t ZnO s u r f a c e s . a) Z n - p o l a r (0001) s u r f a c e , O.225 L e x p o s u r e ; b) n o n p o l a r (5051) s u r f a c e , O.03 L exposure; c) 0 - p o l a r (0001) s u r f a c e , O.0015 L exposure.


210


t h a t were l o o k e d f o r b u t were n o t found i n c l u d e d CO, C 0 , m e t h a n o l , f o r m a l d e h y d e , a c e t a l d e h y d e , e t h a n o l , propane, and 1,3c y c l o h e x a d i e n e . I n g e n e r a l , the temperatures a t w h i c h t h e s e p r o d u c t s were d e t e c t e d were the h i g h e s t on the Z n - p o l a r p l a n e , and l o w e s t on the 0 - p o l a r p l a n e . T a b l e I shows the peak temperatures o f the v a r i o u s p r o d u c t s . These temperatures d i d n o t s h i f t w i t h the coverage of 2 - p r o p a n o l . 2

Table I.

Temperature Programmed D e s o r p t i o n and D e c o m p o s i t i o n Peak Temperatures o f V a r i o u s P r o d u c t s Peak Temperature C


Experiment

Species

(0001)

(5051)

(0001)

H 125 H 0 225,325 propene 125 acetone 125 2 - p r o p a n o l 100

160 130 230 225 150

350 540 350 350 250

H 0 adsorption

H 0

130

230,540

Acetone d e s o r p t i o n

acetone

100

Propene a d s o r p t i o n

propene

90

2-Propanol decomposition

2

2

2

2

100,225,325

100,200

250 t o 300

100

not ads.

B o t h d e h y d r o g e n a t i o n and d e h y d r a t i o n were o b s e r v e d on t h e s e surfaces. On the t h r e e s u r f a c e s s t u d i e d , the acetone peak, w h i c h was a p r o d u c t o f d e h y d r o g e n a t i o n , a l w a y s appeared a t the same temperature a s the propene peak, w h i c h was a p r o d u c t o f d e h y d r a t i o n . The H peak a l s o appeared a t the same temperature as acetone on t h e (0001) and the (000"T) s u r f a c e , b u t was a t a l o w e r temperature on the n o n p o l a r s u r f a c e . The w a t e r peaks were s m a l l , and appeared a t temperatures d i f f e r e n t from the peaks o f the c a r b o n compounds. T a b l e I I l i s t s the peak a r e a s o f the carbon p r o d u c t s f o r the t h r e e s u r f a c e s a t d i f f e r e n t c o v e r a g e s . The a r e a s o f the H peaks were n o t shown because t h e i r a c c u r a c i e s were low due t o the much h i g h e r H background p r e s s u r e s t h a n the c a r b o n compounds. The w a t e r peak a r e a s were a l s o n o t q u a n t i f i e d because a d s o r p t i o n from background w a t e r r e s u l t e d i n w a t e r d e s o r p t i o n a t t h e same temperature a s w a t e r from the r e a c t i o n . S i n c e the amount o f water adsorbed from t h e background was n o t e a s i l y c o n t r o l l a b l e , the w a t e r a r e a s were n o t used. The l o c a t i o n o f the water peaks from the r e a c t i o n was d e f i n i t i v e l y i d e n t i f i e d by u s i n g d e u t e r a t e d 2 p r o p a n o l ((CD^) CD0D). The r e a c t i o n p r o d u c t , w h i c h was D 0 , c o u l d be s e p a r a t e l y measured from the background w a t e r , w h i c h was H 0 . H-D exchange i n the mass s p e c t r o m e t e r ( 3 ) , however, s t i l l made q u a n t i t a t i v e d e t e r m i n a t i o n o f the peak a r e a i n a c c u r a t e . N o n e t h e l e s s , w i t h i n the l a r g e u n c e r t a i n t i e s , the H / a c e t o n e and the H 0 / p r o p e n e r a t i o s were independent o f the 2 - p r o p a n o l c o v e r a g e . No 2

2

2

2

2

2

2

2


12. LUI ET AL.

Table I I .

Temperature-Programmed Decomposition of2-Propanol

Product Quantities i n 2-Propanol Decomposition on ZnO Product Peak Areas, amp-K


Exp.

L

a

Acetone

Propene

b

2-Pr0H

Ac/2-Pr0H >

d

c

Propene/Ac >'

(0001) surface 5.63 O.003 14.07 O.015 16.01 O.03 13.81 O.12 O.225 14-93

O.49 O.94 1.03 1.07 1.19

1.47 3.90 4.51 3.67 4.02

3.86(3.02) 3.61(2.82) 3.50(2.72) 3.76(2.93) 3.72(2.88)

O.087(O.52) O.067(O.40) O.065(O.39) O.077(O.47) O.078(O.47)

(5051) surface 9.23 O.005 16.11 O.015 21.23 O.03 O.06 30.24 O.12 31.27 38.84 O.24 43.41 O.54

O.5 O.89 1.31 1.70 1.62 2.31 2.35

2.05 6.45 10.43 17.82 24.15 32.34 44.88

2.88(2.27) 2.50(1.97) 2.04(1.60) 1.70(1.34) 1.31(1.02) 1.20(O.95) O.97(O.76)

O.053(O.32) O.055(O.33) O.062(O.36) O.057(O.33) O.052(O.28) O.060(O.36) O.055(O.32)

4-45 5.93 13.87 17.91 19.27

1.69(1.45) 1.04(O.89) 1.14(O.98) 1.11(O.96) 1.01(O.87)

(0001) surface 7.52 O.0015 O.006 6.14 15.78 O.3 19.96 1.2 3.6 19.51

a

)

e e e e e

e e e e e

Apparent exposure. The a c t u a l exposure was ten to one hundred times higher due to the dosing action.

k)

Ratio of acetone to 2-propanol.

c

)

Ratio of propene to acetone.

d

)

Values i n the brackets are r a t i o s corrected f o r the mass spectrometer s e n s i t i v i t i e s and pumping speeds. They represent the r a t i o s of the molecular f l u x e s f o r desorbing from the surface.

e

)

These values were small. They were not measured because of the high background s i g n a l s i n these experiments.


211

212


a t t e m p t was made t o o b t a i n t h e s e r a t i o s u s i n g d e u t e r a t e d 2 - p r o p a n o l because the major c r a c k i n g fragments f o r d e u t e r a t e d acetone and d e u t e r a t e d propene were b o t h a t ra/e=46. Acetone and Propene D e s o r p t i o n . D e s o r p t i o n o f adsorbed acetone y i e l d e d o n l y acetone on a l l t h r e e s u r f a c e s . O t h e r p o s s i b l e p r o d u c t s t h a t were s e a r c h e d f o r but n o t found i n c l u d e d H , H 0 , methane, CO and C 0 . The d e s o r p t i o n p r o f i l e s from t h e s e s u r f a c e s a r e shown i n F i g u r e 3 , and the peak t e m p e r a t u r e s a r e l i s t e d i n T a b l e I . A s i n g l e sharp d e s o r p t i o n peak was o b s e r v e d on the 0 p o l a r s u r f a c e . A b r o a d peak was o b s e r v e d on the Z n - p o l a r s u r f a c e w h i c h might be two o v e r l a p p i n g peaks. Two d i s t i n c t peaks were o b s e r v e d f o r t h e n o n p o l a r (5051) and (1010) s u r f a c e s . The a r e a s o f the two peaks i n c r e a s e d b u t a t d i f f e r e n t r a t e s w i t h i n c r e a s i n g exposure. The a r e a o f the h i g h e r temperature peak i n c r e a s e d more r a p i d l y i n i t i a l l y , b u t appeared t o be s a t u r a t e d a t about O.27 L . T h i s s a t u r a t i o n m i g h t be r e a l , b u t i t m i g h t a l s o be due t o i n c r e a s i n g c o m p e t i t i o n from background w a t e r a d s o r p t i o n w i t h i n c r e a s i n g exposure (see n e x t s e c t i o n ) . The a r e a o f the l o w e r temperature peak i n c r e a s e d more s l o w l y , b u t c o n t i n u e d t o i n c r e a s e u n t i l i t s a r e a was more t h a n f i v e t i m e s the a r e a o f the h i g h e r temperature peak a t the h i g h e s t exposure s t u d i e d (20 L ) . The peak t e m p e r a t u r e s were c o n s t a n t . The dependence on the exposure was n o t s t u d i e d on the o t h e r two s u r f a c e s . The TPD p r o f i l e s o f adsorbed propene a r e shown i n F i g u r e 4 f o r the 0 - p o l a r and t h e n o n p o l a r s u r f a c e . A s i n g l e peak was o b s e r v e d on b o t h s u r f a c e s . The peak temperatures a r e l i s t e d i n T a b l e I . O n l y the d e s o r p t i o n o f propene, and no hydrogen, CO, C0 > o r methane were d e t e c t e d . O n l y one exposure was s t u d i e d . On the Z n p o l a r s u r f a c e , exposure up t o 6 L d i d n o t r e s u l t i n d e t e c t a b l e propene d e s o r p t i o n . H , w a t e r , methane, CO, C 0 , benzene, and c y c l o h e x e n e were s e a r c h e d f o r b u t were n o t found. Thus propene appeared n o t t o adsorb on t h i s s u r f a c e . I n f a c t , e t h y l e n e was a l s o found n o t t o adsorb on the Z n - p o l a r s u r f a c e under t h e s e c o n d i t i o n s . 2

2


2

2

2

2

Water D e s o r p t i o n . A f t e r each TPD o r h i g h temperature a n n e a l i n g , background w a t e r was adsorbed onto t h e s u r f a c e d u r i n g c o o l i n g o f the sample. The amount o f w a t e r adsorbed i n c r e a s e d w i t h l o n g e r c o o l i n g t i m e . H e a t i n g o f t h e samples a f t e r c o o l i n g i n vacuo a l w a y s showed desorbed w a t e r a t t e m p e r a t u r e s l i s t e d i n J T a b l e I . The e f f e c t o f adsorbed w a t e r was s t u d i e d on t h e (5051) and the (0001) surface. No e f f e c t was found i n the 2 - p r o p a n o l d e c o m p o s i t i o n . However, i n the acetone a d s o r p t i o n on the (5051) s u r f a c e , i t was found t h a t the h i g h e r temperature acetone d e s o r p t i o n peak decreased w i t h i n c r e a s i n g c o o l i n g t i m e , f o r a g i v e n acetone exposure. The l o w e r temperature peak, however, was n o t a f f e c t e d . Discussion The TPD r e s u l t s o f acetone and propene a r e d i s c u s s e d f i r s t , s i n c e t h e y a r e used f o r the d i s c u s s i o n o f 2 - p r o p a n o l d e c o m p o s i t i o n . Propene a d s o r p t i o n on ZnO powder had been s t u d i e d by TPD and by i n f r a r e d s p e c t r o s c o p y (9-12). A r e v e r s i b l y and an i r r e v e r s i b l y adsorbed propene were found. S i n c e the r e v e r s i b l y adsorbed form



LUI E T A L .


—4

1

100

I

r

*

300 TEMPERATURE

500 C

F i g u r e 3. TPD s p e c t r a o f acetone i n acetone d e s o r p t i o n . a) Z n - p o l a r (0001) s u r f a c e , O.6 L exposure; b) n o n p o l a r (5051) s u r f a c e , O.28 L exposure; c) 0 - p o l a r (0001) s u r f a c e , O.03 L exposure.

X 3

0

200 TEMPERATURE

400 C

Figure 4. TPD s p e c t r a o f propene i n propene a d s o r p t i o n . a) n o n p o l a r (5051) s u r f a c e , 14.4 L exposure; b) 0 - p o l a r (0001 ) s u r f a c e , O.72 L exposure.



214


c o u l d be removed by e v a c u a t i o n a t room temperature (10). i t w o u l d n o t be d e t e c t e d i n our e x p e r i m e n t s . The i r r e v e r s i b l y adsorbed form was found t o e x i s t as a π - a l l y l a n i o n s p e c i e s based on IR s p e c t r o s c o p y (11.12). I t competes w i t h CO f o r a d s o r p t i o n s i t e s (10), i n d i c a t i n g t h a t i t s a d s o r p t i o n i n v o l v e s a Zn i o n . TPD o f t h i s s p e c i e s from powder ZnO showed a peak a t about 100°C (12). T h i s temperature c o r r e s p o n d s w e l l w i t h the d e s o r p t i o n temperature o b s e r v e d i n t h i s s t u d y . T h e r e f o r e the propene d e s o r p t i o n peak o b s e r v e d here must be from the r e c o m b i n a t i o n o f an adsorbed π - a l l y l and hydrogen. D i s s o c i a t i v e a d s o r p t i o n o f propene r e q u i r e s a Zn-0 p a i r s i t e w h i c h i s abundant on the n o n p o l a r s u r f a c e . T h a t the Ο ρ ο l a r s u r f a c e , b u t n o t the Z n - p o l a r s u r f a c e a d s o r b s propene i n t h i s manner s u g g e s t s t h a t s u r f a c e d e f e c t s w h i c h expose the n o n p o l a r s u r f a c e s ( o r o t h e r Zn-0 p a i r s ) e x i s t more a b u n d a n t l y on the 0 - p o l a r t h a n on the Z n - p o l a r s u r f a c e . A d s o r p t i o n o f acetone on ZnO was much l e s s s t u d i e d . R e s u l t s from an i n f r a r e d s p e c t r o s c o p i c s t u d y showed t h a t acetone i s adsorbed i n the form o f an e n o l a t e (13). A t h i g h c o v e r a g e , a p o l y m e r i c s p e c i e s i s formed. The s i n g l e sharp TPD peak o b s e r v e d f o r the 0 - p o l a r s u r f a c e i n d i c a t e d o n l y one type o f adsorbed acetone. The two d i s t i n c t peaks f o r t h e n o n p o l a r s u r f a c e , and the b r o a d peak f o r the Z n - p o l a r s u r f a c e i n d i c a t e d the presence o f d i f f e r e n t t y p e s o f a d s o r p t i o n on t h e s e s u r f a c e s . S i n c e the c o v e r a g e s were low i n t h i s s t u d y , the f o r m a t i o n o f p o l y m e r i c acetone was n o t l i k e l y . The d i f f e r e n t forms may t h e n be due t o two d i f f e r e n t forms o f adsorbed acetone (such as acetone o r e n o l ) , o r two d i f f e r e n t k i n d s o f s u r f a c e s i t e s . Water a p p a r e n t l y reduced the amount o f h i g h e r temperature form o f acetone. I t might be t h a t a d s o r b e d w a t e r reduces the e n o l f o r m a t i o n (which i s assumed t o r e s u l t i n the h i g h e r temperature acetone peak) i n the former e x p l a n a t i o n , o r i t competes f o r a d s o r p t i o n on the h i g h e r temperature s i t e s i n the l a t t e r e x p l a n a t i o n . More d e f i n i t e statements would r e q u i r e a d d i t i o n a l information. The d e c o m p o s i t i o n o f 2 - p r o p a n o l showed b o t h s i m i l a r i t i e s and d i f f e r e n c e s among the s u r f a c e s . The most n o t a b l e s i m i l a r i t y i s the f a c t t h a t propene and acetone were produced a t about the same r a t i o on a l l s u r f a c e s . D e h y d r o g e n a t i o n t o form acetone was the dominant r e a c t i o n , as has been o b s e r v e d on ZnO powders (7). The d e s o r p t i o n temperatures o f the r e a c t i o n p r o d u c t s , a c e t o n e , propene, and hydrogen were a l w a y s h i g h e r t h a n the temperature o f d e s o r p t i o n o f the adsorbed a c e t o n e , propene, and hydrogen (hydrogen does n o t adsorb on ZnO under our c o n d i t i o n s ) . Thus the e v o l u t i o n o f acetone and propene a r e r e a c t i o n l i m i t e d i n 2 - p r o p a n o l d e c o m p o s i t i o n . These, t o g e t h e r w i t h the o b s e r v a t i o n t h a t acetone and propene were a l w a y s e v o l v e d a t the same temperature suggest t h a t acetone and propene a r e formed from a common i n t e r m e d i a t e on the d i f f e r e n t s u r f a c e s , the f o r m a t i o n o r d e c o m p o s i t i o n o f w h i c h i s the r a t e l i m i t i n g s t e p . The e v o l u t i o n o f w a t e r , however, was a t the same temperature as the d e s o r p t i o n o f adsorbed water. Thus the p r o c e s s i s desorption limited. Three i n t e r e s t i n g d i f f e r e n c e s can be e a s i l y i d e n t i f i e d . The f i r s t i s the d i f f e r e n c e i n the temperature o f d e s o r p t i o n from the d i f f e r e n t s u r f a c e s . I n g e n e r a l , e v o l u t i o n o f the d e c o m p o s i t i o n p r o d u c t s a r e a t the l o w e s t temperature on the 0 - p o l a r s u r f a c e ,



12.

LUI E T A L .

Temperature-Programmed Decomposition of'2-Propanol

i n t e r m e d i a t e on the n o n p o l a r s u r f a c e , and the h i g h e s t on the Z n p o l a r s u r f a c e . The o r d e r i s i d e n t i c a l t o t h a t o b s e r v e d f o r the d e c o m p o s i t i o n o f methanol on the same t h r e e s u r f a c e s (3-5)* Thus, the common i n t e r m e d i a t e p o s t u l a t e d above i n t e r a c t s most w e a k l y w i t h the 0 - p o l a r s u r f a c e , and most s t r o n g l y w i t h the Z n - p o l a r s u r f a c e . I n a d d i t i o n t o d i f f e r e n t b o n d i n g c h a r a c t e r i s t i c s o f the r e a c t i o n i n t e r m e d i a t e on the d i f f e r e n t s u r f a c e s , a t l e a s t two o t h e r i n t e r a c t i o n s t h a t d i f f e r on the d i f f e r e n t s u r f a c e s can be i d e n t i f i e d . The a t o m i c arrangement o f an i d e a l Z n - p o l a r s u r f a c e i s such t h a t a l a y e r o f Zn i o n s i s more o u t w a r d l y s i t u a t e d t h a n the n e x t l a y e r o f 0 i o n s . Because the exposed i o n s are Zn i o n s w h i c h a r e n o n p o l a r i z a b l e , t h i s s u r f a c e i s a h a r d a c i d (14)* C o n v e r s e l y , an i d e a l 0 - p o l a r s u r f a c e has a l a y e r o f 0 i o n s more o u t w a r d l y s i t u a t e d t h a n the n e x t l a y e r o f Zn i o n s . These exposed 0 i o n s make the s u r f a c e a s o f t base. The i n t e r m e d i a t e i n 2 - p r o p a n o l d e c o m p o s i t i o n i s an e n o l a t e i o n (13)* B e i n g a b a s e , i t s h o u l d i n t e r a c t more s t r o n g l y w i t h a h a r d a c i d t h a n a s o f t base. The second type o f i n t e r a c t i o n i s d i p o l a r i n t e r a c t i o n . The a t o m i c arrangement o f the s u r f a c e s i s such t h a t a s t r o n g d i p o l e p o i n t i n g outward i s p r e s e n t on the Z n - p o l a r s u r f a c e , and a s t r o n g d i p o l e p o i n t i n g i n w a r d i s p r e s e n t on the 0 - p o l a r s u r f a c e . The n o n p o l a r s u r f a c e , r e l a t i v e l y s p e a k i n g , does n o t p o s s e s s a d i p o l e . The o p p o s i t e o r i e n t a t i o n o f the d i p o l e on the two p o l a r s u r f a c e s w o u l d i n t e r a c t w i t h the d i p o l e o f the e n o l a t e i n t e r m e d i a t e i n an o p p o s i t e manner. T h i s s h o u l d c o n t r i b u t e t o the d i f f e r e n t temperature o f e v o l u t i o n o f the p r o d u c t s . The second d i f f e r e n c e among the s u r f a c e s i s the f a c t t h a t , e x c e p t H2O, the o t h e r t h r e e d e c o m p o s i t i o n p r o d u c t s , H , a c e t o n e , and propene, were e v o l v e d a t the same temperature on the two p o l a r s u r f a c e s , b u t H was e v o l v e d a t a l o w e r temperature on the n o n p o l a r surface. I t i s i n t e r e s t i n g t o compare t h e s e r e s u l t s w i t h the o b s e r v a t i o n s by Koga e t a l . who s t u d i e d the d e c o m p o s i t i o n o f 2 p r o p a n o l a t 100°C on ZnO powder (13)* They found t h a t i f the gas phase 2 - p r o p a n o l was s u d d e n l y removed from the gas phase, the e v o l u t i o n o f hydrogen c o n t i n u e d , b u t the e v o l u t i o n o f acetone stopped. The e v o l u t i o n o f acetone resumed a f t e r r e a d m i s s i o n o f 2 p r o p a n o l . T h i s b e h a v i o r can be e x p l a i n e d by the f a c t t h a t the major exposed f a c e o f t h e i r ZnO powder sample was t h e n o n p o l a r p l a n e . I t i s o n l y on t h i s s u r f a c e t h a t H can be e v o l v e d w i t h o u t c o n c u r r e n t e v o l u t i o n o f acetone i n the absence o f gaseous p r o p a n o l . From the temperature a t w h i c h the d e c o m p o s i t i o n p r o d u c t s e v o l v e d , i t w o u l d seem t h a t the 0 - p o l a r s u r f a c e s h o u l d be the most a c t i v e i n 2 - p r o p a n o l d e c o m p o s i t i o n . However, a c l o s e e x a m i n a t i o n o f the t e m p e r a t u r e s i n T a b l e I shows t h a t on the 0 - p o l a r s u r f a c e , the d e s o r p t i o n temperature o f the m i n o r p r o d u c t w a t e r was a c t u a l l y r a t h e r h i g h - h i g h e r t h a n any o f the p r o d u c t s from the n o n p o l a r surface. Thus i n a s t e a d y s t a t e r e a c t i o n a t t e m p e r a t u r e s b e l o w about 100°C., the 0 - p o l a r s u r f a c e c o u l d be e a s i l y p o i s o n e d by adsorbed w a t e r , l e a v i n g o n l y the n o n p o l a r s u r f a c e a c t i v e . T h a t H was e v o l v e d a t a l o w e r temperature t h a n a c e t o n e and propene on the n o n p o l a r s u r f a c e , b u t n o t on the o t h e r s u r f a c e s , was i n t e r e s t i n g . A p o s s i b l e e x p l a n a t i o n i s t h a t the formation of e n o l a t e (13) from 2 - p r o p a n o l t a k e s p l a c e most r e a d i l y on the n o n p o l a r f a c e because o f the a v a i l a b i l i t y o f Zn-0 p a i r s s u c h t h a t 2

2

2

2


215


216


the conversion of i t into propene and acetone was the slow step. On the p o l a r surfaces, i t s formation was the slow step. I t has been reported that the decomposition of 2-propanol was a s t r u c t u r e - i n s e n s i t i v e reaction on ZnO (15). Our r e s u l t s suggest otherwise. Further work being planned to study steady state reactions on these s i n g l e c r y s t a l surfaces w i l l provide the answer to t h i s discrepancy. The t h i r d difference i s the different r e a c t i v i t y of the surfaces. The f r a c t i o n of adsorbed 2-propanol being decomposed i s i l l u s t r a t e d i n Table I as the r a t i o s of acetone/undecomposed 2propanol. These r a t i o s are the lowest on the 0 - p o l a r surface and the highest on the Zn-polar surface. On the nonpolar surface, the r a t i o s changed from a high value close to those on the Zn surface, to a low value close to those on the 0 surface as the coverage of 2-propanol increased. These data can be explained by the presence of two types of s i t e on these surfaces: a reactive s i t e on which 2propanol decomposes, and an unreactive s i t e on which 2-propanol simply adsorbs and desorbs. The s t i c k i n g coefficients of 2propanol on these two s i t e s vary i n the same manner with coverage on the two p o l a r surfaces, but d i f f e r e n t l y on the nonpolar surface, such that the reactive s i t e i s populated more e a s i l y . An alternate explanation i s that there i s only one type of s i t e for each p o l a r surface. But the s i t e s on the two p o l a r surfaces are different, y i e l d i n g d i f f e r e n t acetone to 2-propanol ratios. Both types of s i t e s are present on the nonpolar surface. I f the i n i t i a l s t i c k i n g coefficient of 2-propanol on the s i t e s s i m i l a r to those found on the Zn surface i s higher, the v a r i a t i o n of the acetone to 2propanol r a t i o i s then explained. However, t h i s l a t t e r model does not automatically explain why the products acetone and propene should desorb at one i d e n t i c a l temperature from the nonpolar plane, that was d i f f e r e n t than those on the p o l a r surfaces unless t h i s difference can be accounted f o r by the different d i p o l a r i n t e r a c t i o n between the surface and the adsorbed intermediate. In conclusion, the chemical properties of ZnO depend on the p a r t i c u l a r surface plane that i s exposed. This surface s p e c i f i c i t y has now been demonstrated for the decomposition of 2-propanol, methanol, formaldehyde and formic a c i d , and adsorption and desorption of acetone, propene, water, CO, and C 0 . These data have made possible better understanding of the r e s u l t s using ZnO powder. I t w i l l be i n t e r s t i n g to seo how different are the c a t a l y t i c properties of these surfaces. 2

Acknowledgment Support of t h i s work by the Petroleum Research Fund administered by the American Chemical Society i s g r a t e f u l l y acknowledged. Literature 1. 2. 3.

Cited

V.E. Henrich, Prog. Surface Sci. 1983, 14, 113. W.H. Cheng, and H.H. Kung, Surface Sci. 1982, 122, 21. S. Akhter, W.H. Cheng, K. Lui, and H.H. Kung, J. Catal. 1984,85, 437.


12. 4. 5. 6. 7. 8. 9. 10. 11. 12.


13. 14. 15.

LUI E T A L .


W.H. Cheng, S. Akhter, and H.H. Kung, J. Catal. 1983, 82, 341. S. Akhter, and H.H. Kung, to be published. M. Bowker, H. Houghton, K.C. Waugh, T. Giddings, and M. Green, J. Catal. 1983, 84, 252. O.V. Krylov, "Catalysis by Nonmetals," Academic Press, 1970. K. Lui, MS thesis, Northwestern University, 1984. A.L. Dent, and R.J. Kokes, J. Amer. Chem. Soc. 1970, 92, 6709, 6718. A.A. Efremov, and A.A. Davydov, Kinet. Catal. 1980, 21, 383. R.J. Kokes, Intra-Science Chem. Rept. 1972, 6, 77. A.A. Davydov, A.A. Yefremov, V.G. Mikhalchenko, and V.D. Sokovskii, J. Catal. 1979, 58, 1; R. Spinicci, and A. Tofanari, J. Thermal Analysis, 1982, 23, 45. O. Koga, T. Onishi, and K. Tamaru, J. Chem. Soc. Faraday Trans. I, 1980, 76, 19. R.G. Pearson, editor: "Hard and Soft Acids and Bases," Dowden, Stroudsburg, 1973. G. Djega-Mariadassou, and L. Davignon, J. Chem. Soc. Faraday Trans. I, 1982, 78, 2447.

RECEIVED

October 4, 1984


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Solid State Chemistry in Catalysis - American Chemical Society

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