Modifications of Surface Reactivity by Structured Overlayers on Metals

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ROBERT J. MADIX Department of Chemical Engineering, Stanford University, Stanford, CA 94305

The dehydrogenation of alcohols has been studied on both copper and nickel surfaces. On clean copper, excepting CH OH, alcohols dehydrogenated to their respective aldehydes via the surface alkoxyl species. Methanol required the presence of surface oxygen to remove hydrogen as water. With the alkoxide identified as the reaction intermediate from these experiments, the dehydrogenation of CH OH was examined on Ni(100) with adsorbed sulfur acting as a reaction modifier. A complete change from total to partial dehydrogenation was noted at 0.38 monolayer of sulfur. This change in selectivity was due to stabilization of the methoxyl group by the sulfur, and to an increase in the activation barrier for C-H bond cleavage. Since the methoxyl group formed H CO at a higher temperature, and the barrier for C-H bond rup ture increased, formaldehyde desorbed rather than dehydrogenated. Using vibrational spectroscopy and CO as a probe of the surface, the site responsible for CH O formation was identified with the four-fold hollow. 3

3

2

3

S e l e c t i v e poisoning i s widely employed i n c a t a l y t i c processes. This s e l e c t i v e poisoning i s achieved by t r e a t i n g the c a t a l y s t s with carbon, c h l o r i n e , s u l f u r and/or oxygen-containing compounds to modify the r e a c t i v i t y of the c a t a l y s t s . The methods of surface science o f f e r the opportunity to c l a r i f y the chemistry r e s p o n s i b l e for these e f f e c t s . Metal surfaces with w e l l - d e f i n e d overlayers of r e a c t i o n modifiers can be prepared, and the k i n e t i c s and mechanism of r e a c t i o n s can be d e f i n i t i v e l y s t u d i e d , e l u c i d a t i n g both the elementary steps and t h e i r rate c o n s t a n t s . In t h i s paper the e f f e c t s of modifying n i c k e l s i n g l e - c r y s t a l surfaces with s u l f u r on the dehydrogenation of methanol i s d i s c u s s e d . The r e s u l t s c l e a r l y

0097-6156/84/0248-0057$06.00/0 © 1984 American Chemical Society

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

58

CATALYTIC MATERIALS

i l l u s t r a t e that surface modifiers can a l t e r the s t a b i l i t y of t i o n intermediates and thereby change s e l e c t i v i t y .

reac-

Experimental The t o o l s used f o r the experiments described below have been desc r i b e d i n s e v e r a l books and review a r t i c l e s (1-3)* Surface s t r u c ture i s determined by low energy e l e c t r o n d i f f r a c t i o n (LEED), surface composition by Auger e l e c t r o n spectroscopy (AES), and r e a c t i o n k i n e t i c s and mechanism by temperature programmed r e a c t i o n spectroscopy (TPRS). Standard u l t r a - h i g h vacuum technology i s used to maintain the surface i n a w e l l - d e f i n e d s t a t e . As t h i s a r t i c l e i s a c o n s o l i d a t i o n of p r e v i o u s l y published work, d e t a i l s of the experiments are not discussed here. Results and D i s c u s s i o n In order to recognize the p a t t e r n of r e a c t i v i t y of alcohols on modified n i c k e l s u r f a c e s , i t i s e s s e n t i a l to know the r e a c t i o n pathways e x h i b i t e d by l e s s r e a c t i v e s u r f a c e s . I n i t i a l l y the dehydrogenation of CH OH was studied on copper (4) and s i l v e r (5) single c r y s t a l surfaces. On C u ( l l O ) , following the preadsorption of submonolayer q u a n t i t i e s of atomic oxygen, methanol reacted v i a the f o l l o w i n g sequence ( 4 , 6 ) : 3

CH 0H,.

> CH-OH,.

o

3

(g)

3

^ ( a )

+

V )

Œ

+

° (a)

3

Œ

( « )

CH 0 3

2 H

(a)

( a )

(a)

>

H

(1)

Œ

3°(.)

+

> H CO 2

( g )

+ H

(

+

a

H

° (a)

H

2°(g)

(

2

)

(

3

)

(4)

)

H

(

> 2(g)

5

)

This r e a c t i o n sequence was d e f i n i t i v e l y shown by use of temperature programmed r e a c t i o n spectroscopy ( 7 ) · The key to the success of t h i s method was that r e a c t i o n (4) was the r a t e - l i m i t i n g s t e p , allowing p o s i t i v e i d e n t i f i c a t i o n of the CH3 0 ( ) intermediate by TPRS. Isotopic s u b s t i t u t i o n with 0 and deuterium was used to i d e n t i f y steps (2) and ( 3 ) . With the s t a b i l i t y of t h i s intermediate e s t a b l i s h e d , i t s s p e c t r a l features i n photoemission and high r e s o l u t i o n e l e c t r o n energy l o s s ( v i b r a t i o n a l ) spectroscopy (EELS) could be determined. Indeed, with u l t r a v i o l e t photoelectron spectra (UPS) i t was shown that methanol reacted with the preadsorbed oxygen to a


4.

MADIX

59

Structured Overlayers on Metals

form CH30( ) on C u ( l l O ) w i t h the C-0 a x i s perpendicular to the s u r f a c e ( 6 ^ 8 ) . On clean copper ( i n the absence of oxygen), subsequent to methanol a d s o r p t i o n o n l y methanol was observed to desorb. With the s p e c t r a l f i n g e r p r i n t s of CH30( ) e s t a b l i s h e d , however, i t was determined that on clean copper a

a

< reversible 3

>

+

( 6 )

3 (a;

(a;

Thus, upon h e a t i n g , CH30( ) and H ( ) recombination o c c u r r e d , and no hydrogen atom recombination was observed. P a r e n t h e t i c a l l y , i t i s i n t e r e s t i n g to note that no other a l c o h o l s show t h i s completely r e v e r s i b l e a d s o r p t i o n on copper. E t h a n o l , n-propanol, i - p r o p a n o l , n-butanol and ethylene g l y c o l a l l dehydrogenate to t h e i r r e s p e c t i v e aldehydes on c l e a n C u ( l l O ) (9,10). Temperature programmed r e a c t i o n s p e c t r a f o r those a l c o h o l s are shown i n F i g u r e 1. In general the a l c o h o l s show s e v e r a l peaks. The lowest two TPRS peaks f o r the a l c o h o l i t s e l f correspond to d e s o r p t i o n of the a l c o h o l s from m u l t i l a y e r and monolayer coverages, r e s p e c t i v e l y , w h i l e the peak observed at the highest temperature i s due to recombination of adsorbed a l k o x y l species w i t h adsorbed hydrogen atoms (peaks at 175, 220, 300 Κ f o r isopropanol m/e45, f o r example). In the case of ethylene g l y c o l the dehydrogenation product i s the dialdehyde (CH0) . A summary of the r e s u l t s w i t h the a l c o h o l s i s g i v e n i n Table I. With the exception of ethylene g l y c o l which bonds v i a both f u n c t i o n a l groups, the s t a b i l i t y of the a l k o x y l group can be c o r r e l a t e d w i t h the C-H bond s t r e n g t h on the carbon i n the «-position w i t h respect to the h y d r o x y l oxygen ( 9 ) ; t h i s bond must be broken to form the aldehyde. On n i c k e l , a more r e a c t i v e surface than copper, the r e a c t i v i t y of methanol i s somewhat d i f f e r e n t . Temperature programmed r e a c t i o n spectroscopy produces o n l y CO and hydrogen products sub sequent to CH 0H a d s o r p t i o n on Ni(100) or N i ( l l l ) s u r f a c e s , each product being evolved from the surface i n a d e s o r p t i o n - l i m i t i n g s t e p ( l l ) . EELS r e s u l t s on both N i ( l l l ) ( 1 2 ) and Ni(100)(13) c l e a r l y show the formation of CH3 0 ( ) at low temperatures (see Table I I ) . However, the dehydrogenation a c t i v i t y i s h i g h enough to break the C-H bonds below 300 K, producing adsorbed CO and atomic hydrogen which desorb as CO and H at 355 Κ and 445 K, r e s p e c t i v e l y . In other words the C-H bond i s e a s i l y cleaved, and the r e a c t i o n proceeds toward adsorbed atomic hydrogen. As s u l f u r i s added to the Ni(100) s u r f a c e , the p a t t e r n of r e a c t i v i t y changes. The temperature programmed r e a c t i o n s p e c t r a , a p o r t i o n of which i s shown i n F i g u r e 2, c l e a r l y r e f l e c t s the formation of H C0, H and CO i n a common r a t e - l i m i t i n g step ( r l s ) . This s t e p , the dehydrogenation of the methoxyl s p e c i e s , becomes apparent as the s u l f u r coverage ( e ) approaches 0.25 monolayer. With i n c r e a s i n g s u l f u r coverage the peak temperature of the products formed by the r e a c t i o n sequence a

a

2

3

a

2

2

2

g



F i g u r e 1. Temperature-programmed r e a c t i o n s p e c t r a (10) f o r a l c o h o l d e h y d r o g e n a t i o n on C u ( 1 1 0 ) f o l l o w i n g a d s o r p t i o n a t 140 K. T h e s e s p e c t r a show m o l e c u l a r d e s o r p t i o n o f t h e a l c o h o l f r o m a c o n d e n s e d l a y e r ( l o w e s t t e m p e r a t u r e p e a k , a w e a k l y bound s t a t e t o t h e m e t a l ( m i d d l e a l c o h o l p e a k ) , and f r o m r e c o m b i n a t i o n o f a l k o x y l g r o u p s with adsorbed hydrogen. A l d e h y d e f o r m a t i o n f r o m t h e a l k o x y l g r o u p i s c l e a r l y shown.


5

3

2

2

2

at 200 Κ

Decomposes to C H 3 C H O , H and C H C H 0 H at 348 Κ

2

2

C H 0

7

3

Some desorbs, r e s t d i s s o c i a t e s

3

2

2

iso-C H OH

3

7

3

Decomposes to C H C H C H 0 , H and C H C H C H 0 H at 340 Κ

n-C H 0

3

7

Desorption of chemisorbed m a t e r i a l at 245 K, with some d i s s o c i a t i o n

n-C H OH

7

2

Desorption of physisorbed m a t e r i a l at 180 Κ

3

5

n-C H OH

at 200 Κ

Decomposes to C H 3 C H O , H and CH3 CH2 OH at 348 Κ

2

rest dissociates

at 200 Κ

C H 0

2

2

Decomposes to form H C 0 + H + CH3OH at 355 Κ

Some desorbs, r e s t d i s s o c i a t e s

TPRS Products

14

14

6X10

8X10

5X10

c

)

14

14

(

c

1 4

)

1 4 ( d )

(

1 4

14

1 4

10 X 1 0

-50X10

6X10

10x10

6X10

11X10

Coverage (species cm" )

Continued on next

Summary of Reactions of A l c o h o l s on C u ( l l O )

Some desorbs,

( b )

( b )

I.

C^OH

3

CH 0

3

CH OH

Adsorbed Species

Table


2

(CH OH)

2

2

2

Decomposes to (CH0) , H and (CH 0H) a t 390 Κ 2

Desorption of chemisorbed m a t e r i a l at 300 K, w i t h d i s s o c i a t i o n

Values f o r the maximum a l l k o x y coverage obtained w i t h 1/4 monolayer pre-dosed oxygen.

Multilayer desorption.

(d)

2

cm" )>; u n c e r t a i n t y i n estimate 20%.

(c)

14

From r e f . (6)

(5.5X10

14

14

4X10

14

6X10

14

14

40X10

5X10

8X10

5X10

H C0, χ + 2 (a)

H

(a)

(7)

C 0

( 8 )

> 2 (g)

C 0

>

CO, ν (a;

> CO, , (g;

2 H

ν (a)

o

(a)

2 H

(a)

+

( 9 )

°°(.)

(10)

H

(

> 2(g)

U

)

s h i f t s to higher temperature, r e v e a l i n g the increased s t a b i l i t y of the methoxy group i n the presence of s u r f a c e s u l f u r . The a c t i v a t i o n energy f o r r e a c t i o n (7) i n c r e a s e s by 6 kcal/gmol f o r s u l f u r coverages between 0.20 and 0.46 monolayer. Note that the r a t e l i m i t i n g step i s the C-H bond cleavage. The dramatic change from t o t a l to p a r t i a l dehydrogenation above 0 = 0.20 i s i l l u s t r a t e d i n F i g u r e 3. Above 0 « 0.32 the CO produced goes to z e r o . The r e s i d u a l mass spectrometer s i g n a l at m/e = 28 above t h i s s u l f u r coverage can be accounted f o r e n t i r e l y as a c r a c k i n g f r a c t i o n of H C0. The b u i l d u p of H C0 a t g

g

2

2

Table I I . V i b r a t i o n a l Spectra of Methoxyl Species on Metal Surfaces

frequency i n cm""* 16

mode

Cu(100)

v(M-O)

290(s)

405(m)

v(C-O)

1010(s)

1040(s)

980(s)

6(CH )

1450 (w)

1440 (m)

1455 (m)

v(C-H

2910(m)

2955 (m)

2966 (m)

3

Ni(lll)

Ni (100)


CATALYTIC MATERIALS

64

Mass 30

ΔΡ

ι , ι ι

Mass 28

ι !

d J ^ /

b.

/ / \;\ Χ.

ι 350

F i g u r e 2.

400

! ! ι

1

X0.2

^1

450 Temperature (K)

1

ι

550

500

Temperature-programmed r e a c t i o n spectra for H CO 2

and CO from CH^OH as a f u n c t i o n of preadsorbed sulfur. (a)

n

g

= 3.2X10 14

4.8X10 atoms/cm.

1 4

(b)

4.2X10 ,

(c)

14 and (e) 7.4X10 Hydrogen a l s o desorbs i n a peak at (d)

5.1X10

the same temperature as H C0 and CO. The 2

simultaneous

e v o l u t i o n of H C0 (m/e = 30), H , 2

2

and C0(m/e = 28) i s i n d i c a t i v e of a common r a t e l i m i t i n g step i d e n t i f i e d as C ^ O ^ ) dehydro genation.


4.

MADIX



65

66

CATALYTIC MATERIALS

the expense of CO i s c l e a r . This t r a d e - o f f occurs as the methoxi d e becomes more s t a b l e and the a c t i v a t i o n b a r r i e r f o r C-H bond cleavage i n c r e a s e s . This i n c r e a s e i s the r e s u l t of a decrease i n the metal-hydrogen bond s t r e n g t h w i t h i n c r e a s i n g s u l f u r coverage. I f the binding energy (BE) of H2CO were constant as o i n c r e a s e d , i t i s apparent why the s e l e c t i v i t y must change. Since r e a c t i o n (7) occurs at i n c r e a s i n g temperatures the surface l i f e t i m e f o r H 2 C 0 ( ) must decrease markedly; thus i t has l e s s time to react to CO and H . Furthermore, the a c t i v a t i o n b a r r i e r f o r C-H bond cleavage (step (9)) increases a l s o . Consequently step (8) dominates step (9) w i t h i n c r e a s i n g θ . In f a c t t h i s s e l e c t i v i t y change i s f u r t h e r enhanced as the b i n d i n g energy of H C0 to the surface a c t u a l l y decreases w i t h i n c r e a s i n g Q ( 1 1 ) * The change i n s e l e c t i v i t y and r e a c t i v i t y can be r e l a t e d to the s t r u c t u r e of s u l f u r on the s u r f a c e . The two-dimensional s t r u c t u r e on Ni(100) i s w e l l known ( 1 4 ) . Two ordered s t r u c t u r e s form under the circumstances of these experiments; they are shown i n F i g u r e 4. As the s u l f u r coverage approaches 0.25 monolayer the p(2X2) s t r u c t u r e (Figure 4a) develops. Between 0.25 and 0.50 monolayer the f o u r - f o l d hollow p o s i t i o n s i n the center of the p(2X2) s u l f u r u n i t c e l l are f i l l e d u n t i l the c(2X2) s t r u c t u r e forms (Figure 4b). Between q = 0.25 and 0.50 the s t a b i l i t y of CH3O (and consequently the s e l e c t i v i t y to H C0) i n c r e a s e s , but the t o t a l number of b i n d i n g s i t e s f o r the methoxyl species decreases. Thus the t o t a l r e a c t i v i t y f o r H C0 formation goes through a maximum. Note that the " s p e c t a t o r " s u l f u r i n the c(2X2) p o s i t i o n blocks a s i t e f o r CH 0 formation, but s t a b i l i z e s CH3O toward f u r t h e r dehydrogenation. The s i t e r e s p o n s i b l e f o r CH3O formation can be i d e n t i f i e d w i t h the use of CO as a probe molecule. As i n c r e a s i n g amounts of s u l f u r are added to N i ( 1 0 0 ) , the d e s o r p t i o n s t a t e c h a r a c t e r i s t i c of CO on the c l e a n s u r f a c e disappears, and two new s t a t e s appear at 315 and 380 K, r e s p e c t i v e l y . These s t a t e s p e r s i s t from about S =0.15 to 0 O.46 (see F i g u r e 5 ) . In t h i s coverage range one s u l f u r atom blocks a d s o r p t i o n of one CO molecule (15). In order to determine the nature of the s i t e f o r C H 0 ( ) f o r m a t i o n , CO was adsorbed i n v a r y i n g amounts i n the β s t a t e p r i o r to exposure to methanol a t q = 0.38 to see i f t h i s b i n d i n g of CO to t h i s s i t e would block methanol d i s s o c i a t i o n (15). The r e s u l t s showed a l i n e a r decrease i n I^CO production w i t h CO precoverage (15 ). S a t u r a t i o n of the 00(β ) s t a t e o n l y completely blocked methanol dehydrogenation. The b i n d i n g of t h i s CO was examined f u r t h e r w i t h EELS. The v i b r a t i o n a l l o s s s p e c t r a are shown i n F i g u r e 6 f o r CO adsorbed to s a t u r a t i o n coverage at 90 Κ on Ni(100) w i t h v a r y i n g degrees of p r e s u l f i d i n g . Corresponding temperature programmed d e s o r p t i o n s p e c t r a are shown on the r i g h t hand s i d e of the f i g u r e . At 0 = 0.37 three CO binding s t a t e s were i d e n t i f i e d . These correspond to CO i n f o u r - f o l d n i c k e l hollows i n the p(2X2) s t r u c t u r e (1750 cm" ), two-fold b r i d g i n g s i t e s on p(2X2) (1935 cm" ) and f o u r - f o l d n i c k e l hollows on the c(2X2) s t r u c t u r e (2115 cm" ). P a r t i a l desorption experiments g

a

2

2

s

2

2

3

g

g

3

χ

χ

g

1

1

1


a

4.

MADIX

67


c F i g u r e 4.

Real space s t r u c t u r e s f o r (b)

Ni(100)c(2X2)S

and

(a) Ni(100)p(2X2)S, (c) one intermediate

coverage (see the t e x t ) .


68

CATALYTIC MATERIALS

L

X0.4

8

2xlO" omp

β

X

ΔΡ

200

250

300

350

Temperature

Figure 5.

400

450

500

(K)

CO/CO desorption s p e c t r a as a f u n c t i o n of s u l f u r concentration. each case,

CO was adsorbed to s a t u r a t i o n i n

(a) n 14

(f) (h)

8.0X10

(b) 1.5 Χ 1 0

(d) 3.2 Χ 1 0 ,

5.1X10 , 14

- 0,

1 4

(c) 2.3X10 , 14

g

1 4

,

(e) 4.8 Χ 1 0

1 4

,

14

(g) 7.4X10 , and 2

atoms/cm .

The -peak a

charac

t e r i s t i c of b i n d i n g of CO to clean Ni(100) i s d i s p l a c e d by s u l f u r . peaks,

3^

and 32*

In i t s place two d e s o r p t i o n appear.


4.

MADIX

69


CO(sat)/[Ni(100) + S (0 = 0 - 0.5)] T = 90 Κ S

A

|370

X330

J 0

ι

ι

ι

1000 2 0 0 0 3 0 0 0 Energy L o s s ( c m " ) 1

F i g u r e 6.

I Li

ι

ι

ι

ι—

100 2 0 0 3 0 0 4 0 0 500 T e m p e r a t u r e (K)

V i b r a t i o n a l l o s s s p e c t r a f o r CO adsorbed on Ni(100) w i t h s u l f u r coverages between zero and 0.50 monolayers.

For 0

g

= 0.37 three b i n d i n g

s t a t e s are evident (see t e x t ) .


70

CATALYTIC MATERIALS

showed that the v i b r a t i o n a l l o s s 1750 cm"" o r i g i n a t e d from the 00(3^ state. C l e a r l y then formation of the C H 0 ( ) i s a s s o c i a t e d with the f o u r - f o l d hollow s i t e s on the p a r t i a l l y s u l f i d e d s u r f a c e . This explains the decrease i n o v e r a l l a c t i v i t y with s u l f u r a d s o r p t i o n . 3

a

Summary Model s t u d i e s of methanol dehydrogenation on Ni(100) show that dramatic changes i n s e l e c t i v i t y can be achieved by preadsorbing s u l f u r i n f o u r - f o l d hollow s i t e s . The s e l e c t i v i t y changes r e s u l t from increases i n the a c t i v a t i o n b a r r i e r f o r C-H bond cleavage. This change causes a sharp d i f f e r e n t i a t i o n i n competing steps subsequent to the i n i t i a l C-H bond rupture of the surface methoxyl s p e c i e s . Desorption of H C0 i s overwhelmingly favored over f u r t h e r dehydrogenation with i n c r e a s i n g s u l f u r c o n c e n t r a t i o n . 2

Literature 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16.

Cited

Ertl, G.; Kupers, J. "Low Energy Electrons and Surface Chemistry"; Verlag Chemie: Weinheim, 1974. Madix, R.J. Adv. in Catalysis 1980, 29, 1. Roberts, M.W. Adv. in Catalysis 1980, 29, 55. Wachs, I.E.; Madix, R.J. J. Catalysis 1978, 53, 208. Wachs, I.E.; Madix, R.J. Surf. Sci. 1978,76, 531. Bowker, M.; Madix, R.J. Surf. Sci. 1980, 95, 190. Madix, R.J. in "Chemistry and Physics of Solid Surfaces"; Vol. II, Vanselow R., Ed., p.63. Hofmann P.; Mariani C.; Horn, K.; Bradshaw A.M. "Proceedings of the 3rd European Conference on Surface Science"; Degras, D.A.; Costa, M. Eds., Vol. 1, 1980, 720. Bowker, M.; Madix, R.J. Surf. Sci. 1982, 116, 549. Jorgensen, S.; Madix, R.J. Surf. Sci. 1983 130, L291. Johnson, S.W.; Madix, R.J. Surf. Sci. 1981 103, 361. Demuth, J.E.; Ibach, J. Chem. Phys. Lett. 1979, 60, 395. Madix, R.J.; Gland, J.L. To be published. Perereau, M.; Oudar, J. Surf. Sci. 1970, 20, 80. Madix, R.J.; Lee, S.B.; Thornburg, M. J. Vac. Sci. Tech. 1983, A1, 1254. Sexton, B.A. Surf. Sci. 19979, 88, 299.

R E C E I V E D December 19, 1983


Modifications of Surface Reactivity by Structured Overlayers on Metals

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