Catalyst Characterization Science - American Chemical Society

15

Downloaded by HONG KONG UNIV SCIENCE TECHLGY on April 4, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch015

Oxygen Interactions and Reactions on Palladium(100): Coadsorption Studies with C2H4, H2O, and CH3OH Ε. M.Stuve1,S. W. Jorgensen, and R. J. Madix Department of Chemical Engineering, Stanford University, Stanford, CA 94305 The reactions of ethylene, water, and methanol with coadsorbed oxygen on Pd(100) were studied with temper ature programmed reaction spectroscopy (TPRS) and high resolution electron energy loss spectroscopy (EELS). Ethylene dehydrogenation was poisoned by oxygen, and direct hydrogen transfer reactions between water and oxygen and between methanol and oxygen were observed. These reactions demonstrate the Brönsted base role of adsorbed oxygen perviously found on Ag(110) and show further that more active t r a n s i t i o n metals which themselves activate C-H bonds c a t a l y t i c a l l y oxidize via a two-step mechanism i n which the surface i n t e r mediates are scavenged by adsorbed oxygen. Recent studies /1,2/ have shown t h a t the surface reactivity of A g ( l l O ) c a n be g r e a t l y e n h a n c e d by p r e a d s o r b e d o x y g e n . The b e h a v i o r o f o x y g e n w i t h o t h e r c o a d s o r b a t e s on A g ( l l O ) c a n be c l a s s i f i e d into three categories: (1) Lewis a c i d or t h r o u g h - s u r f a c e interaction in w h i c h an o x y g e n a t o m w i t h d r a w s c h a r g e f r o m t h e s u r f a c e a n d c r e a t e s one o r more e l e c t r o n e g a t i v e s i t e s e l s e w h e r e on t h e s u r f a c e t h a t favor t h e a d s o r p t i o n o f charge d o n a t i n g s p e c i e s , (2) Brflnsted base i n t e r a c t i o n i n w h i c h an e l e c t r o n e g a t i v e o x y g e n a t o m c a n a b s t r a c t an a c i d i c p r o t o n from a n o t h e r c o a d s o r b e d s p e c i e s , and (3) n u c l e o p h i l i c a t t a c k in which the electrons of the oxygen atom attack an electron d e f i c i e n t s i t e on a n o t h e r c o a d s o r b e d s p e c i e s . It i s therefore of i n t e r e s t t o know w h e t h e r t h e s e f o r m s o f o x y g e n i n t e r a c t i o n s o c c u r on o t h e r m e t a l s , i n p a r t i c u l a r , on t h o s e more r e a c t i v e t h a n s i l v e r , s u c h as t h e g r o u p V I I I t r a n s i t i o n m e t a l s . The h i g h e r r e a c t i v i t y o f t h e s e m e t a l s p r e s e n t s an a d d e d c o m p l i c a t i o n i n t h e s t u d y o f o x y g e n i n t e r a c t i o n s i n t h a t t h e c o a d s o r b e d s p e c i e s i s u s u a l l y c a p a b l e o f some s o r t of reaction without oxygen. F o r r e a c t i o n s on A g ( 1 1 0 ) , o x y g e n a c t s a s a promoter, but i n t h e i n s t a n c e s where r e a c t i o n can o c c u r without o x y g e n , o x y g e n may a l t e r o r e v e n p o i s o n a r e a c t i o n , o r p e r h a p s p r o mote an e n t i r e l y different reaction path ( i n which oxygen may directly participate). The l a t t e r c a s e i s a n a l o g o u s t o oxidation 1Current address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Federal Republic of Germany 0097-6156/85/0288-0165$06.00/0

© 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


166

CATALYST CHARACTERIZATION SCIENCE

r e a c t i o n s on A g ( l l O ) . R e a c t i o n m o d i f i c a t i o n o r p o i s o n i n g may i n v o l v e the aforementioned oxygen interactions, b u t an a d d i t i o n a l effect, known a s s c a v e n g i n g , i s p o s s i b l e . A g e n e r a l d e s c r i p t i o n o f oxygen scavenging i s as f o l l o w s . S u r f a c e i n t e r m e d i a t e s f o r m e d by i n t e r a c t i o n w i t h t h e m e t a l may r e a c t i n s t e a d w i t h a s c a v e n g e r ( o x y g e n ) a n d thereby p r o h i b i t o r a l t e r subsequent s u r f a c e r e a c t i o n s which would occur i n t h e absence o f t h e scavenger. I n t h i s summary n o t e , we d i s c u s s t h e r e a c t i o n s o f C 2 H 4 , H 2 0 , a n d CH3OH w i t h o x y g e n o n P d ( 1 0 0 ) . L e w i s a c i d o x y g e n i n t e r a c t i o n s a r e o b s e r v e d w i t h C2H4 t h a t c o n t r i b u t e to t h e poisoning of the dehydrogenation reaction; Brflnsted base i n t e r a c t i o n s between oxygen a n d H 2 0 a n d CH3OH, i n w h i c h h y d r o g e n i s d i r e c t l y t r a n s f e r r e d t o an oxygen a t o m , p r e d o m i n a t e ; scavenging of h y d r o g e n p r o d u c e d f r o m C2H4 d i s s o c i a t i o n by a d s o r b e d o x y g e n a c c o u n t s f o r o x i d a t i o n o f C2H4. Experimental T h e e x p e r i m e n t s w e r e p e r f o r m e d i n a n u l t r a - h i g h vacuum c h a m b e r ( b a s e pressure = 5 X 1 0 ~ n Torr) with f a c i l i t i e s f o r high resolution e l e c t r o n e n e r g y l o s s s p e c t r o s c o p y ( E E L S ) , t e m p e r a t u r e programmed r e a c t i o n s p e c t r o s c o p y (TPRS), Auger s p e c t r o s c o p y , and low energy e l e c t r o n d i f f r a c t i o n ( L E E D ) , w h i c h h a s been p r e v i o u s l y d e s c r i b e d / 3 / . The r e a c t a n t s w e r e d o s e d t h r o u g h g l a s s , m u l t i - c a p i l l a r y a r r a y , m o l e c u l a r beam d o s e r s w h i c h g a v e a n e f f e c t i v e beam p r e s s u r e o f b e t w e e n 2 0 0 ( f o r O2) a n d 2 0 0 0 ( f o r C?HA) t i m e s t h a t o f t h e b a c k g r o u n d p r e s s u r e w h i c h was h e l d t o about 2 X 1 0 " 1 0 T o r r d u r i n g t h e d o s e . The P d ( 1 0 0 ) s a m p l e was c l e a n e d by p r o l o n g 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 t r e a t m e n t s / 3 , 4 / a n d s u r f a c e c l e a n l i n e s s w a s v e r i f i e d b y T P R S , E E L S , a n d LEED / 3 , 5 / . The EELS measurements w e r e p e r f o r m e d o n l y i n t h e s p e c u l a r d i r e c t i o n w i t h a n i n c i d e n t beam e n e r g y o f a b o u t 1 e V . The h e a t i j i g r a t e f o r t h e thermal desorption measurements was a b o u t 15 K s " 1 . Additional e x p e r i m e n t a l d e t a i l s c a n be f o u n d e l s e w h e r e / 3 , 6 , 7 / . Results ETHYLENE ADSORPTION AND REACTION The

adsorption

ability ly

involved

adsorption presented H2

TPRS

covered age. C-H

C2H4;

in detail curves

structure

of

02Η4

Additional

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H2

of

C2H4

is

our attention

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H2

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clean

H2 m o l e c u l e s for

f o r oxygen

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coverages

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oxygen

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oxygen

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oxygen

the

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in

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is

t h e C2H4 a n d

on c l e a n

independent

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on P d ( 1 0 0 )

differences

the

the

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desorption

TPRS e x p e r i m e n t s

of

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between

molecules,

0 . 2 4 ML o f

0 . 0 ML

illustrates being

of

Figure

coverage

b u t t h e amount

The

mechanism

/6,8/.

There

cannot

Pd(100)

We f o c u s

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on

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than

Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

to

atoms. 0 . 2 5 ML

15.

Oxygen Interactions and Reactions on Palladium(lOO)

STUVE E T A L .

1

τ

1

1

H , 0 H IC H^ 2


2

u

2

1

1

167

1

O + Pd(100) θ (Μϋ 0

χ1/5

—I

100

1

0.25

1 300

I

I

5 00

I

L_

700

TEMPERATURE ( K )

F i g u r e 1. T e m p e r a t u r e p r o g r a m m e d r e a c t i o n s p e c t r a o f C2H4 a n d H 2 f o l l o w i n g s a t u r a t i o n c o v e r a g e o f C2H4 on t h e c l e a n a n d o x y g e n c o v e r e d P d ( 1 0 0 ) s u r f a c e a t 100 Κ · A t o m i c o x y g e n was g e n e r a t e d on t h e s u r f a c e by e x p o s u r e t o 0 2 a t 300 K.


168



showed t h a t t h e h y d r o g e n y i e l d ( w h i c h i n c l u d e s h y d r o g e n t h a t d e s o r b s i n t h e f o r m o f H2O, s e e b e l o w ) d e c r e a s e s l i n e a r l y w i t h i n c r e a s i n g oxygen c o v e r a g e . T h e H 2 d e s o r p t i o n i n f i g . 1 i s t h e r e s u l t o f C2H4 d e h y d r o g e n a tion C2H4(a) — 2 H2(g) + 2 C(a) (1) T h i s was t h e o n l y r e a c t i o n d e t e c t e d f o r C2H4 on c l e a n P d ( 1 0 0 ) ; no CH4, C2H2, C2H5, o r C5H5 was d e t e c t e d . The e f f e c t o f i n c r e a s i n g oxygen c o v e r a g e i s , t h e r e f o r e , t o p r o g r e s s i v e l y p o i s o n t h e d e h y d r o genation r e a c t i o n . The m a g n i t u d e o f t h e d e c r e a s e i n h y d r o g e n y i e l d w i t h i n c r e a s i n g oxygen c o v e r a g e i n d i c a t e s t h a t two adsorbed oxygen atoms block t h e r e a c t i o n o f o n e C2H4 m o l e c u l e a c c o r d i n g t o t h e stoichiometry of equation (1). T h i s c a n be i n t e r p r e t e d a s a s i t e blocking effect of oxygen. Oxygen i s adsorbed i n t o the p(2x2) s t r u c t u r e f o r c o v e r a g e s b e t w e e n 0 . 0 5 a n d 0 . 2 5 ML / 3 / , a n d e a c h o x y g e n a t o m may b l o c k f o u r Pd a t o m s , a s s u m i n g t h a t 0 r e s i d e s i n t h e f o u r fold hollow / 9 / . A s t w o 0 a t o m s b l o c k t h e r e a c t i o n o f o n e C2H4 m o l e c u l e , i t f o l l o w s t h a t a n e n s e m b l e o f e i g h t i s o l a t e d Pd a t o m s a r e required f o r e a c h C2H4 r e a c t i o n e v e n t . Oxygen has an a d d i t i o n a l , m i n o r e f f e c t on C2H4 d e h y d r o g e n a t i o n , t h a t b e i n g a l i m i t e d a b i l i t y t o s c a v e n g e some o f t h e r e a c t i o n p r o d u c t s . S m a l l amounts ( l e s s t h a n 0 . 0 4 ML e a c h ) o f H 2 0 a n d C 0 2 w e r e d e t e c t e d a t 3 3 0 Κ a n d 5 6 5 K , r e s p e c t i v e l y , f r o m C2H4 a n d a p p r o x i m a t e l y 0 . 2 ML o f c o a d s o r b e d o x y g e n /10/. T h e s e s i d e r e a c t i o n s h a v e l i t t l e e f f e c t on C2H4 d e h y d r o g e n a t i o n ; t h e o c c u r a n c e o f t h i s r e a c t i o n d e p e n d s on t h e i n i t i a l , a d s o r b e d s t a t e o f C2H4 a s d i s c u s s e d b e l o w . E l e c t r o n e n e r g y l o s s s p e c t r a f o r C2H4 o n c l e a n a n d o x y g e n c o v e r e d P d ( 1 0 0 ) a r e shown i n f i g . 2 . The v i b r a t i o n a l s p e c t r u m f o r C2H4 on c l e a n P d ( 1 0 0 ) ( f i g . 2 a ) i s a s f o l l o w s / 6 , 8 / : v(CH) = 2980 c m " 1 , « ( C H ) » 1 4 5 5 c m ' 1 , v ( C C ) = 1 1 3 5 c m * 1 , CHo d e f o r m a t i o n s = 9 2 0 c m " 1 , a n d C2H4 r e s t r i c t e d t r a n s l a t i o n = 3 9 0 c m " * . S i m i l a r l y , t h e a s s i g n ments f o r t h e v i b r a t i o n a l s p e c t r u m o f C2H4 on t h e o x y g e n covered s u r f a c e ( f i g . 2b) a r e / 1 0 / : v i C H ) = 3 0 2 0 c m " 1 , « ( C H ) = 1510 c m " 1 , a n d C H d e f o r m a t i o n s = 9 4 0 c m " 1 . T h e v ( C C ) mode was f o u n d a t 1 3 4 0 cm f o r C2D4 o n o x y g e n c o v e r e d P d ( 1 0 0 ) / 1 0 / . The f r e q u e n c i e s o f t h e v i b r a t i o n a l l y c o u p l e d δ CH2) a n d v ( C C ) modes e n a b l e u s t o a s s i g n t h e s p e c t r u m on c l e a n P d ( 1 0 0 ) t o d i - o - b o n d e d o r a p p r o x i m a t e l y s p ^ h y b r i d i z e d C2H4 a n d t h e s p e c t r u m on t h e p ( 2 X 2 ) 0 s t r u c t u r e t o π - b o n d e d o r approximately s p 2 h y b r i d i z e d C2H4. The p r o p e r a s s i g n m e n t o f t h e 6 ( C H p ) a n d v ( C C ) modes i s n o t t r i v i a l , a s we h a v e d i s c u s s e d e l s e w h e r e /6,8,10,11/. 2

2

2

The TPRS a n d E E L S r e s u l t s a r e i n t e r p r e t e d a s f o l l o w s . The H 2 d e s o r p t i o n y i e l d , a m e a s u r e o f t h e e x t e n t o f C2H4 d e h y d r o g e n a t i o n , d e c r e a s e s l i n e a r l y w i t h i n c r e a s i n g oxygen c o v e r a g e , and t h e EELS r e s u l t s show t h a t C2H4 i s d i - o - b o n d e d on t h e c l e a n s u r f a c e a n d π - b o n d e d on t h e o x y g e n c o v e r e d s u r f a c e . Thus, i t i s t h e d i - o - b o n d e d form o f C2H4 t h a t u n d e r g o e s d e h y d r o g e n a t i o n . The s a t u r a t i o n c o v e r a g e o f π b o n d e d C2H4 i s a p p r o x i m a t e l y 0 . 2 5 ML a n d i s i n d e p e n d e n t o f o x y g e n coverage, whereas the saturation of di-o-bonded C2H4 decreases



15.

STUVEETAL.


169

Figure 2. E l e c t r o n energy l o s s spectra f o r s a t u r a t i o n coverage of C2H4 on c l e a n ( a ) a n d o x y g e n c o v e r e d ( b ) P d ( 1 0 0 ) a t 8 0 K . Oxygen was d o s e d a s i n f i g . 1 t o a c o v e r a g e o f 0 . 1 8 M L .



170 linearly

from

to

ML.

0.25

coverage

i s estimated H20


Temperature OH

was

obtained Κ

state

groups the due at

AND

with for

0.0

to

255

K,

the

is

not

on

17/

oxygen

coverage

detected

in

is

fig.

increased 2b

as

its

of

H20

ML.)

spectra

Pd(100)

adsorption at

167

bound This

Κ

show

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to

represents

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•

H20(g)

these

H20

shown

clean

the

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directly

2 OH EELS r e s u l t s according to

0.035

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OH

H20

H20

ML

C2H4

be o n l y

oxygen

0^ s t a t e to

and 0 ( f i g .

to

programmed

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182

ML

(Di-o-bonded

REACTIONS OF

peaks,

0.12

OH

in

H20,

reaction

fig.

surface and

surface

Curve

(a)

and c o n t a i n s

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3.

the

/7/.

following the

the

0^ s t a t e

An

coadsorption

reaction

of

OH

of

H?0

groups

III

+ 0 groups

(2) form

at

about

+ 0 + 2 OH

Equation (2) d e s c r i b e s t h e d i s p r o p o r t i o n a t i o n t h e Ύ r e a c t i o n peak i n f i g . 3b c o u l d a l s o be via

at

additional

175

Κ

(3) o f OH g r o u p s , although due t o OH d i s s o c i a t i o n

OH > 0 + H OH + Η + H 2 0 ( g )

(4a) (4b)

W h e t h e r t h e OH g r o u p s r e a c t v i a e q u a t i o n (2) o r (4) c a n be t e s t e d w i t h TPRS m e a s u r e m e n t s f o l l o w i n g t h e f o r m a t i o n o f OH g r o u p s , formed f r o m H 2 0 and 0 c o a d s o r p t i o n , w i t h h e a t i n g t o 200 K , by c o a d s o r p t i o n o f D a t o m s a s shown i n c u r v e s ( c ) a n d ( d ) o f f i g . 3 . The γ s t a t e f o r H 2 0 i s s e e n a t 230 Κ i n c u r v e ( c ) , w h i l e t h e D 2 p e a k a t 360 Κ ( t h e normal t e m p e r a t u r e f o r r e c o m b i n a t i v e d e s o r p t i o n o f D2) verifies that D atoms were i n d e e d c o a d s o r b e d . C u r v e ( d ) shows t h a t _no HD0 d e s o r b e d at t h e t e m p e r a t u r e o f t h e y s t a t e i n ( c ) , but t h a t a h i g h e r t e m p e r a t u r e s t a t e , l a b e l l e d 3 , o c c u r r e d a t 330 K. The 3 H 2 0 s t a t e i s d u e t o t h e r e a c t i o n o f a t o m i c oxygen and h y d r o g e n t o f o r m H 2 0 as v e r i f i e d i n s e p a r a t e t h e r m a l d e s o r p t i o n measurements o f c o a d s o r b e d 0 and H atoms /7,9/. The H i n t h e 3 HDO s t a t e came f r o m t h e a d s o r p t i o n o f h y d r o g e n f r o m t h e r e s i d u a l g a s i n t h e vacuum c h a m b e r a s t h e r e a c t a n t surface was b e i n g p r e p a r e d (note the high m u l t i p l i e r of curve ( d ) ) . Thus, t h e a b s e n c e o f HDO i n t h e λ s t a t e r u l e s o u t e q n . ( 4 ) s o t h e OH r e a c tion must proceed through the direct hydrogen transfer mechanism given in eqn. (2). T h a t t h e 3 H 2 0 s t a t e i s n o t s e e n i n f i g . 3b shows t h a t H 2 0 d o e s n o t d i s s o c i a t e t o OH a n d H on o x y g e n c o v e r e d P d ( 1 0 0 ) . A t no t i m e , t h e n , d u r i n g t h e t e m p e r a t u r e ramp f o l l o w i n g H?0 and 0 c o a d s o r p t i o n a r e Η a t o m s a d s o r b e d on t h e s u r f a c e , s o t h e f o r m a t i o n of OH g r o u p s must a l s o be a d i r e c t t r a n s f e r o f h y d r o g e n a s d e p i c t e d i n eqn. (3).


15.

STUVEETAL.


171

H 0/OH/0/D/Pd(100) Downloaded by HONG KONG UNIV SCIENCE TECHLGY on April 4, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch015

2

HDO/OH + D

H 0. D / O H * D 2

2

H 0/H 0 • 0 2

2

(b)

H 0/H 0 2

2

(a) 100

200

400

300

Temperature (K) Figure 3 . T e m p e r a t u r e programmed r e a c t i o n s p e c t r a f o r t h e w a t e r a n d o x y g e n c o a d s o r p t i o n s y s t e m on P d ( 1 0 0 ) . (a): H 2 0 desorption f o l l o w i n g a n e x p o s u r e o f 2 L H 2 0 a t 100 K . (b): H20 desorption f o l l o w i n g e x p o s u r e o f t h e s u r f a c e w i t h 0 . 1 6 ML a t o m i c o x y g e n t o 2 L HpO a t 100 K . (c): H 2 0 a n d Do d e s o r p t i o n f r o m a s u r f a c e w i t h c o a d s o r b e d OH a n d D s p e c i e s . The OH g r o u p s w e r e g e n e r a t e d b y a d s o r b i n g 0 . 1 0 ML a t o m i c o x y g e n a t 300 K , 2 L H 2 0 a t 100 K , a n d h e a t i n g t o 200 K. The s u r f a c e w i t h 0 . 2 0 ML o f OH g r o u p s was e x p o s e d t o 2 L Do a t 100 K . (d): HDO d e s o r p t i o n f r o m a s u r f a c e p r e p a r e d a s i n ( c ) , e x c e p t t h a t t h e D e x p o s u r e was 4 L . 2


172



METHANOL REACTIONS I n t h i s s e c t i o n we p r e s e n t p r e l i m i n a r y r e s u l t s o f CH3OH r e a c t i o n s with preadsorbed oxygen. Of p r i m a r y i n t e r e s t i s t h e mechanism f o r t h e removal o f t h e h y d r o x y l hydrogen d u r i n g f o r m a t i o n o f t h e methoxy (CH3O) i n t e r m e d i a t e . F i g . 4 shows TPRS c u r v e s f o r CH3OH, H 2 0 , a n d H 2 C 0 f o l l o w i n g c o a d s o r p t i o n o f CH3OH a n d 0 on P d ( l O O ) . Desorption of CO a n d C 0 2 a l s o o c c u r s , b u t we l i m i t t h e d i s c u s s i o n h e r e t o w a t e r f o r m a t i o n by CH3OH and 0 . Methanol d e s o r p t i o n o c c u r s i n t h r e e peaks a t 155 K , 180 K , a n d 205 K . The 150 Κ p e a k i s a s s i g n e d t o m u l t i l a y e r s o f CH3OH, a n d t h e t w o h i g h e r t e m p e r a t u r e s t a t e s a r e d u e t o CH3OH in d i r e c t contact with the surface. T h e r e a r e two peaks o b s e r v e d f o r m/e = 30 a t 155 Κ a n d 215 K . We a s s i g n t h e 155 Κ peak t o t h e c r a c k i n g f r a c t i o n o f CH3OH, b u t t h e 215 peak m u s t be d u e t o f o r m a l d e h y d e (H2C0) d e s o r p t i o n , s i n c e i t i s s l i g h t l y h i g h e r i n t e m p e r a t u r e and o f g r e a t e r a m p l i t u d e t h a n t h e 205 Κ peak f o r CH3OH. Water d e s o r p t i o n o c c u r s i n t h r e e p e a k s a t 155 K , 215 K , a n d 3 3 0 K . As o f t h i s w r i t i n g we a r e n o t c e r t a i n o f t h e s o u r c e o f H 2 0 t h a t d e s o r b s a t 155 K , b u t b e l i e v e i t t o be e i t h e r f r o m w a t e r i n t h e r e s i d u a l g a s , o r a s m a l l i m p u r i t y i n t h e CH3OH s a m p l e . We f o c u s more c l o s e l y on t h e 215 Κ a n d 3 3 0 Κ H 2 0 d e s o r p t i o n s t a t e s w i t h t h e r e s u l t s shown i n f i g . 5 . Curves ( a ) - ( c ) were o b t a i n e d f o l l o w i n g t h e c o a d s o r p t i o n o f CHoOD a n d 0 and show t h a t none o f t h e d e u t e r i u m l a b e l f o r m s w a t e r i n t n e 330 Κ s t a t e . We c o n c l u d e t h a t t h e H 2 0 d e s o r p t i o n a t 330 Κ i n c u r v e ( c ) i s due t o t h e m e t h y l hydrogens r e a c t i n g w i t h preadsorbed oxygen. The 3 3 0 Κ peak i s t h e 3 H 2 0 s t a t e f o r 0 and Η r e c o m b i n a t i o n ( s e c t i o n 3 . 2 ) , so i t i s c l e a r t h a t t h e methyl hydrogens were t r a n s f e r r e d t o t h e s u r f a c e at a t e m p e r a t u r e l o w e r t h a n 330 K. The p r i m a r y r o u t e by w h i c h t h e d e u t e r i u m label i s r e m o v e d i s t h e d e s o r p t i o n o f HD0 o r D 2 0 a t 215 K , a n d we a s s i g n t h i s p e a k t o t h e d i r e c t t r a n s f e r o f d e u t e r i u m f r o m CH3OD t o e i t h e r o x y g e n o r a n h y d r o x y l g r o u p w i t h t h e e n s u i n g f o r m a t i o n o f CH3O species CH3OD + 0 CH3O + 0D CH3OD + 0D > CH3O + D 2 0 CH3OD + OH • CH3O + HDO

(5a) (5b) (5c)

F o r m a t i o n o f C H 3 O , f o l l o w i n g CH3OH a n d 0 c o a d s o r p t i o n a n d a n n e a l i n g t h e s a m p l e t o 1/5 K , was v e r i f i e d w i t h E E L S m e a s u r e m e n t s ; t h e s p e c t r a w e r e i n g o o d a g r e e m e n t w i t h EEL s p e c t r a o f CH3OH on c l e a n P d ( 1 0 0 ) /12/. The c o n c u r r e n t d e s o r p t i o n o f H 2 C 0 a t 2 1 5 Κ i n d i c a t e s t h a t o n e o f t h e m e t h y l h y d r o g e n s was l o s t a t o r b e l o w t h i s t e m p e r a t u r e a n d i s apparently the source of Η incorporated i n t o the water products at 215 Κ i n c u r v e s 5b a n d 5c a c c o r d i n g t o C H 3 0 + 0 • H 2 C 0 + OH CH3O + OH + H 2 C 0 + H 2 0 CH3O + OD > H 2 C 0 + HDO

(6a) (6b) (6c)

N o t e t h a t r e a c t i o n ( 6 a ) i s t h e s o u r c e o f OH f o r r e a c t i o n ( 5 c ) . In c u r v e s ( d ) - ( f ) o f f i g . 5 , we s e e t h a t t h e r e i s no e f f e c t upon e i t h e r



15.

stuveetal.


173

Products/CH3OH+ O+PtldOO)

150

200

250 Temperature

300

350

(K)

Figure 4. T e m p e r a t u r e p r o g r a m m e d r e a c t i o n s p e c t r a o f CH3OH, H 2 C 0 and HoO f o l l o w i n g a d s o r p t i o n o f m u l t i l a y e r s o f CH3OH on P d ( l O O ) a t The o x y g e n was 130 t p r e c o v e r e d w i t h 0 . 2 5 ML a t o m i c o x y g e n . a d s o r b e d a t 300 K . A l l t r a c e s a r e on t h e same s c a l e .


174


1

1

1

r

Water / Methanol + 0+D + Pd (100) (d)-(f):CH 0H + 0


3

—I

—I

150

200

1

I

250

300

L

25

Temperature (K) Figure 5. Temperature programmed reaction spectra of water d e s o r p t i o n f o l l o w i n g m e t h a n o l a n d o x y g e n c o a d s o r p t i n on P d ( 1 0 0 ) . (a)-(c): D 2 0 , HDO, a n d H 2 0 d e s o r p t i o n f o l l o w i n g a d s o r p t i o n o f m u l t i l a y e r s o f CH3OD a t 1 3 0 Κ on t h e s u r f a c e p r e c o v e r e d w i t h 0 . 2 5 ML a t o m i c o x y g e n , (d)-(f): S o l i d l i n e s a r e t h e same a s ( a ) - ( c ) e x c e p t t h a t CH3OH was u s e d i n p l a c e o f CH3OD. The dashed l i n e s i n curves ( d ) - ( f ) demonstrate t h e e f f e c t o f coadsorbed D atoms. For t h e s e e x p e r i m e n t s t h e o x y g e n p r e c o v e r e d s u r f a c e was e x p o s e d t o 4 L D 2 a t 130 Κ p r i o r t o CH3OH e x p o s u r e .


15.

STUVEETAL.


t h e 155 Κ o r 2 1 5 Κ H 2 0 d e s o r p t i o n s t a t e s when d e u t e r i u m a t o m s a r e c o a d s o r b e d w i t h CH3OH a n d 0. T h e o n l y d i f f e r e n c e s a r e t h e a p p e a r a n c e o f t h e HDO a n d D 2 0 3 s t a t e s a t 3 3 0 Κ d u e t o t h e D a t o m s . Thus, the w a t e r t h a t d e s o r b s a t 2 1 5 Κ was f o r m e d b y a d i r e c t t r a n s f e r o f h y d r o gen f r o m m e t h a n o l t o t h e p r e a d s o r b e d o x y g e n . A t t h e p r e s e n t t i m e we do n o t know t h e i d e n t i t y o f t h e s u r f a c e s p e c i e s r e s p o n s i b l e f o r t h e 215 Κ H 2 0 p e a k . I t i s due t o e i t h e r a s t a b i l i z e d form o f m o l e c u l a r H 0, o r t o a n OH g r o u p more r e a c t i v e t h a n t h a t found f r o m H 0 + 0 ( \ + 20H(aj. B o t h TPRS a n d EELS m e a s u r e m e n t s ( n o t s h o w n ) r u l e o u t OH g r o u p s i d e n t i c a l t o t h o s e formed from H 0 and 0 i n t h i s c a s e . We a r e c u r r e n t l y working t o resolve t h i s question. 2


175

2

a

2

Discussion The results presented above demonstrate that t h e Brflnsted base behavior of adsorbed oxygen, d i s c u s s e d p r e v i o u s l y by B a r t e a u and M a d i x f o r A g ( 1 1 0 ) / l / , a l s o m a n i f e s t s i t s e l f on more a c t i v e t r a n s i t i o n metal s u r f a c e s . We o b s e r v e , i n a d d i t i o n , t h e a b i l i t y o f o x y g e n t o scavenge r e a c t i o n p r o d u c t s . The L e w i s a c i d i n t e r a c t i o n i n v o l v e s c h a r g e w i t h d r a w a l f r o m t h e s u r f a c e by a d s o r b e d oxygen w h i c h s t a b i l i z e s c h a r g e d o n a t i n g s p e c i e s a s i n t h e c a s e o f c o a d s o r b e d CoH*. Ethylene, h o w e v e r , may a d s o r b i n t o e i t h e r o f t w o f o r m s o n Pd(lOO). The d i - o - b o n d e d f o r m o f C 2 H 4 c a n be c l a s s i f i e d a s c h a r g e w i t h d r a w i n g with charge going from t h e s u r f a c e i n t o t h e empty, anti-bonding π-orbital of C2H4 resulting i n rehybridization. The π-bonded form o f C 2 H 4 c a n be c l a s s i f i e d a s c h a r g e d o n a t i n g w i t h c h a r g e g o i n g f r o m t h e f i l l e d , bonding π-orbital o f C2H4 t o t h e s u r f a c e . The EELS s p e c t r u m o f f i g . 2b shows t h a t t h e c h a r g e d o n a t i n g ττ-bonded C 2 H 4 i s i n d e e d s t a b l e w i t h t h e charge w i t h d r a w i n g atomic oxygen, t h o u g h , a c c o r d i n g t o t h e C 2 H 4 TPD f r o m t h e c l e a n a n d p ( 2 X 2 ) 0 s u r f a c e s , t h e b i n d i n g e n e r g y does not change a p p r e c i a b l y . On t h e o t h e r h a n d , i f L e w i s a c i d oxygen i n t e r a c t i o n s s t a b i l i z e charge donating species, then they should d e s t a b i l i z e charge withdrawing s p e c i e s . T h i s i s m a n i f e s t e d by the oxygen-induced poisoning o f C2H4 dehydrogenation which proceeds from d i - o - b o n d e d C 2 H 4 . The i n h i b i t i o n o f C 2 H 4 d e h y d r o g e n a t i o n i s due to the poisoning of sites required f o r di-o-bonded C2H4 adsorption, and t h e r e q u i r e m e n t of eight Pd a t o m s f o r e a c h d i - o - b o n d e d C2H4 molecule indicates that a l a r g e e n s e m b l e i s n e e d e d f o r C-H bond activation. We p r o p o s e t h a t b o t h s i t e b l o c k a g e a n d t h r o u g h - s u r f a c e , Lewis acid i n t e r a c t i o n s between oxygen and d i - o - b o n d e d C2H4 are responsible f o r the oxygen-induced poisoning o f C2H4 dehydrogenation on P d ( 1 0 0 ) . The

direct

hydrogen t r a n s f e r r e a c t i o n s , e q n s . (3) and ( 5 a ) , f o r w i t h 0 a r e examples o f t h e Brflnsted base b e h a v i o r o f oxygen. I n t h e s e r e a c t i o n s , t h e B r f l n s t e d a c i d s p e c i e s , HoO o r CH3OH, g i v e s up i t s a c i d i c h y d r o g e n t o t h e B r f l n s t e d b a s e s p e c i e s , oxygen. The Brflnsted base b e h a v i o r o f oxygen i s not observed with C2H4 b e c a u s e C 2 H 4 i s a much w e a k e r a c i d t h a n e i t h e r H 2 0 o r CH3OH / 2 / . The s c a v e n g i n g e f f e c t o f a t o m i c o x y g e n , o b s e r v e d w i t h C 2 H 4 a n d CH3OH, d o e s n o t d e p e n d i n a d i r e c t way on t h e n a t u r e o f t h e c o a d s o r b a t e , b u t o n l y on t h e v a r i o u s s p e c i e s e v o l v e d d u r i n g t h e r e a c t i o n o f the coadsorbate. Oxygen i s c a p a b l e o f s c a v e n g i n g h y d r o g e n and c a r b o n produced by C 2 H 4 d e c o m p o s i t i o n t o form H 2 0 i n t h e 3 s t a t e a t 330 Κ

H 0 a n d CH3OH 2



176 and

CO a n d C 0 2 a t h i g h e r

believed self. of


the

to

have

Oxygen

adsorbed

temperatures

no d i r e c t

effect

s c a v e n g i n g may p l a y

CH3O,

hydroxyl

however.

hydrogen

These

side reactions are

on t h e i n i t i a l

/10/·

decomposition i t -

an i m p o r t a n t

We h a v e

observed

o f CH3OH t o f o r m

role

i n the s t a b i l i t y

that

oxygen

abstracts

desorbs

a t 215 K,

H 2 0 , which

and t h e m e t h y l h y d r o g e n s t o f o r m H 2 0 a t 330 K. Oxygen s c a v e n g i n g o f the hydroxyl H c a n a c t t o s t a b i l i z e CH3O s p e c i e s b y p r e v e n t i n g t h e CH3O r e h y d r o g e n a t i o n r e a c t i o n . T h i s e f f e c t was p r e v i o u s l y s e e n b y G a t e s a n d K e s m o d e l f o r CH3OH a n d 0 on P d ( l l l ) / 1 3 / . U n l i k e P d ( l l l ) , h o w e v e r , CH3O s p e c i e s a r e s t a b l e o n c l e a n P d ( l O O ) / 1 2 / , a n d we a r e c u r r e n t l y s t u d y i n g w h e t h e r t h e r e i s a n y a d d i t i o n a l s t a b i l i t y o f CH3O on P d ( 1 0 0 ) d u e t o t h e p r e s e n c e o f o x y g e n . Conclusions The

Lewis

acid

and Brflnsted base oxygen

on A g ( 1 1 0 ) a l s o o c c u r o n P d ( 1 0 0 ) . actions ing

were

inferred

o f t h e C2H4

bonded

C0H4.

Brflnsted CH3OH

t o oxygen

tified

species

atomic

hydrogen

base

reaction

acid

found inter-

of the poison-

by d é s t a b i l i s a t i o n o f d i - σ -

interactions

were

observed

between

and 0 as e v i d e n c e d by t h e d i r e c t t r a n s f e r

t o form

from

Lewis

C0H4 a n d 0 a s t h e r e s u l t

dehydrogenation

and 0 a n d between hydrogen

between

interactions previously

Through-surface,

CH3OH.

OH s p e c i e s Oxygen

p r o d u c e d b y C0H4

o

r

from

of

H 2 0 and an a s y e t u n i d e n

a d s o r b e d on P d ( 1 0 0 ) C H

H20

also

scavenges

3 ° decomposition.

Acknowledgments We w i s h t o t h a n k D r . C . R . B r u n d l e o f IBM R e s e a r c h - S a n J o s e f o r t h e l o a n o f t h e vacuum chamber u s e d i n t h i s w o r k . One o f u s ( S W J ) g r a t e fully acknowledges a f e l l o w s h i p from t h e N a t i o n a l Science Founda tion. T h i s w o r k was s u p p o r t e d b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n (NSF-CPE 8320072).

L i t e r a t u r e Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

M. A. Barteau and R. J. Madix, in "The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis," Vol. 4, Eds. D. A. King and D. P. Woodruff (Elsevier, Amsterdam, 1982). M. A. Barteau and R. J. Madix, Surface Science 120 (1982) 262. E. M. Stuve, R. J. Madix, and C. R. Brundle, Surface Science , in press (CO Oxidation on Pd(100)). A. Ortega, F. M. Hoffman, and A. M. Bradshaw, Surface Science 119 (1083) 79. R. J. Behm, K. Christmann, G. Ertl, and M. A. Van Hove, J. Chem. Phys. 73 (1980) 2984. Ε. M. Stuve and R. J. Madix, Surface Science, in press. Ε. M. Stuve, S. W. Jorgensen, and R. J. Madix, submitted to Sur face Science (Water and Oxygen Coadsorption on Pd(100)). Ε. M. Stuve, R. J. Madix, and C. R. Brundle, Surface Science, Proc. ECOSS-6, in press. C. Nyberg and C. G. Tenstal, Surface Science 126 (1983) 163. Ε. M. Stuve and R. J. Madix, submitted to Surface Science. Ε. M. Stuve and R. J. Madix, to be submitted. K. Christmann and J. E. Demuth, J. Chem. Phys. 76 (1982) 6318. J. A. Gates and L. L. Kesmodel, J. Catalysis 83 (1083) 437.

RECEIVED

June 21, 1985


Catalyst Characterization Science - American Chemical Society

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