Studies of the Kinetics and Mechanisms of Ammonia Synthesis and

14 Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces

Downloaded by FUDAN UNIV on December 20, 2016 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch014

A . J . Gellman, M. Asscher, and G . A . Somorjai Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Department of Chemistry, University of California—Berkeley, Berkeley, C A 94720

We have studied the ammonia synthesis reaction over Fe and Re single c r y s t a l surfaces and the hydrodesulfurization of thiophene over the Mo(100) single c r y s t a l surface. The studies have been performed using UHV surface science tools with the c a p a b i l i t y of exposing the surfaces to high pressure, high temperature reaction conditions. The ammonia synthesis reaction was shown to be extremely sensitive to surface structure on both Fe and Re, favoring surfaces with a rough or open topography. The HDS reaction on the Mo(100) surface has been shown to be s i m i l a r to that on MoS2 and appears to proceed via a reaction path that does not produce a strong Mo-S bond as an intermediate species.

The use o f s u r f a c e s c i e n c e t e c h n i q u e s as probes f o r the s t u d y o f c a t a l y t i c p r o c e s s e s was f o r many y e a r s hampered by the l a c k o f d i r e c t c o n n e c t i o n between the g r e a t volume o f i n f o r m a t i o n o b t a i n e d under UHV c o n d i t i o n s and s t u d i e s performed under the h i g h p r e s s u r e , h i g h temperature c o n d i t i o n s o f i n d u s t r i a l c a t a l y s i s . There are v e r y few t e c h n i q u e s f o r s t u d y i n g s u r f a c e s w h i l e e x p o s i n g them t o h i g h p r e s s u r e s o f gases, w h i l e on the o t h e r hand t h e r e are a l a r g e number o f probes o f the surface/vacuum i n t e r f a c e . Our approach t o b r i d g i n g the gap between t h e s e two extremes o f c o n d i t i o n s has been to d e v e l o p a system a l l o w i n g d i r e c t t r a n s f e r o f samples between a h i g h p r e s s u r e c a t a l y t i c environment and the UHV c o n d i t i o n s under w h i c h i t i s p o s s i b l e t o p r e p a r e , s t u d y and c h a r a c t e r i z e t h e i r s u r f a c e s and a d s o r b a t e s on t h e s e s u r f a c e s U s i n g t h i s approach we have been a b l e t o i n v e s t i g a t e a number o f c a t a l y t i c problems on b o t h s i n g l e c r y s t a l and p o l y c r y s t a l l i n e samples. I n t h i s paper we w i l l d i s c u s s the r e s u l t s t h a t we have o b t a i n e d s t u d y i n g two i m p o r t a n t p r o c e s s e s , the s y n t h e s i s o f ammonia over Fe and Re c a t a l y s t s and the h y d r o d e s u l f u r i z a t i o n o f thiophene on Mo. These examples i l l u s t r a t e two problems i n c a t a l y s i s t o which o u r methods are p a r t i c u l a r l y s e n s i t i v e . The f i r s t i s the e f f e c t o f the s t r u c t u r e o f the c a t a l y s t on r e a c t i o n r a t e s . The s e n s i t i v i t y o f the 0097-6156/85/0288-0154S06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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G E L L M A N ET AL.

Ammonia Synthesis and Hydrodesulfurization

155

r e a c t i o n r a t e t o the n a t u r e o f the c a t a l y t i c s u r f a c e i s e x h i b i t e d t o an extreme e x t e n t by the ammonia s y n t h e s i s r e a c t i o n on both Fe and Re. The second i s the e f f e c t on t h i o p h e n e h y d r o d e s u l f u r i z a t i o n o f the a d d i t i o n o f s u l f u r t o t h e s u r f a c e o f a Mo s i n g l e c r y s t a l . Ammonia s y n t h e s i s i s one o f the o l d e s t i n d u s t r i a l c a t a l y t i c p r o c e s s e s and has been s t u d i e d by a l a r g e number o f i n v e s t i g a t o r s . Most work has been performed u s i n g Fe based c a t a l y s t s and i t has been shown t h a t the r a t e l i m i t i n g s t e p i n t h i s r e a c t i o n i s the d i s s o c i a t i v e c h e m i s o r p t i o n o f n i t r o g e n (2). Other work has shown t h a t n i t r o g e n adsorbs p e r f e r e n t i a l l y on the Fe(111) s u r f a c e (5) and the s u g g e s t i o n has been made t h a t i t i s the s e v e n - c o o r d i n a t e atoms i n the second l a y e r below the s u r f a c e t h a t a r e indeed most a c t i v e i n the s y n t h e s i s (4)· By comparison v e r y l i t t l e work has been done u s i n g Re as a c a t a l y s t . Here we r e p o r t the r e s u l t s o f some o f the work t h a t has been completed i n o u r l a b o r a t o r y s t u d y i n g ammonia s y n t h e s i s over b o t h Fe and Re s i n g l e c r y s t a l s u r f a c e s . The extreme s e n s i t i v i t y o f the r e f o r m i n g c a t a l y s t s t o p o i s o n i n g by s u l f u r has s t i m u l a t e d much work s t u d y i n g the h y d r o d e s u l f u r i z a t i o n r e a c t i o n and h y d r o t r e a t i n g r e a c t i o n s i n g e n e r a l . The c a t a l y s t s f o r t h i s r e a c t i o n a r e based on supported molybdenum s u l f i d e s . We have chosen t o study t h i s r e a c t i o n by u s i n g s i n g l e c r y s t a l molybdenum s u r f a c e s and t o combine UHV s u r f a c e s c i e n c e w i t h s t u d i e s of the h y d r o d e s u l f u r i z a t i o n o f thiophene a t h i g h p r e s s u r e s . Experimental The a p p a r a t u s e s used f o r t h e s t u d i e s o f both ammonia s y n t h e s i s and h y d r o d e s u l f u r i z a t i o n were almost i d e n t i c a l , c o n s i s t i n g o f a UHV chamber Dumped by both i o n and o i l d i f f u s i o n pumps to base p r e s s u r e s o f 1x10 T o r r . Each chamber was equipped w i t h Low Energy E l e c t r o n D i f f r a c t i o n o p t i c s used t o determine t h e o r i e n t a t i o n o f the s u r f a c e s and t o a s c e r t a i n t h a t the s u r f a c e s were indeed w e l l - o r d e r e d . The LEED o p t i c s doubled as r e t a r d i n g f i e l d a n a l y z e r s used f o r Auger E l e c t r o n Spectroscopy. I n a d d i t i o n , each chamber was equipped w i t h a UTI 100C quadrupole mass s p e c t r o m e t e r used f o r a n a l y s i s o f background gases and f o r Thermal D e s o r p t i o n S p e c t r o s c o p y s t u d i e s . Each UHV chamber was equipped w i t h a c e l l t h a t c o u l d be s e a l e d o v e r the c r y s t a l a l l o w i n g i t t o be exposed t o h i g h gas p r e s s u r e s , s i m u l a t i n g r e a c t i o n c o n d i t i o n s . The c r y s t a l s were spot welded between two m e t a l p o s t s a l l o w i n g them t o be heated r e s i s t i v e l y both f o r the purpose of c l e a n i n g i n vacuum and f o r h e a t i n g w h i l e exposed to r e a c t a n t gas m i x t u r e s . C r y s t a l temperatures were monitored u s i n g thermocouples spot welded t o t h e i r edges. The h i g h p r e s s u r e c e l l s were connected t o gas h a n d l i n g l i n e s made o f 0.25" s t a i n l e s s s t e e l t u b i n g , forming a c l o s e d l o o p , b a t c h r e a c t o r through which r e a c t a n t gases were c i r c u l a t e d . Samples o f the gas m i x t u r e were taken p e r i o d i c a l l y f o r a n a l y s i s v i a a gas s a m p l i n g v a l v e . The ammonia s y n t h e s i s m i x t u r e was a n a l y z e d f o r ammonia content u s i n g a photoi o n i z a t i o n d e t e c t o r (energy = 10.2 e V ) . The p r o d u c t s o f the HDS o f t h i o p h e n e were separated on a gas chromatograph and a n a l y z e d w i t h a flame i o n i z a t i o n detector. 2 Fe, Re and Mo samples were t y p i c a l l y o f about 1 cm a r e a by 0.5 mm t h i c k and were spark c u t from s i n g l e c r y s t a l r o d s . These were s u b s e q u e n t l y p o l i s h e d u s i n g s t a n d a r d m e t a l l u r g i c a l t e c h n i q u e s and

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

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CATALYST CHARACTERIZATION SCIENCE

f i n a l l y cleaned i n UHV u s i n g v a r i o u s treatments i n c l u d i n g Ar+ i o n s p u t t e r i n g , and both o x i d a t i o n and r e d u c t i o n a t e l e v a t e d t e m p e r a t u r e s . The d e t a i l s o f sample p r e p a r a t i o n and o f the p r o c e d u r e s used t o perform r e a c t i o n s have been d e s c r i b e d t h o r o u g h l y elsewhere (6-9)·


Ammonia S y n t h e s i s K i n e t i c s o v e r Fe C r y s t a l S u r f a c e s . A t y p i c a l ammonia s y n t h e s i s r e a c t i o n was performed a t 5 atm. o f Ν and 15 atm. o f H^ a t a temperature o f 736K o v e r an F e ( l 1 l ) s u r f a c e . A f t e r a p e r i o d d u r i n g w h i c h t h e ammonia formed was adsorbed on t h e w a l l s o f t h e h i g h p r e s s u r e loop i t s a c c u m u l a t i o n r a t e i n t h e r e a c t o r was c o n s t a n t . T h e c o r r e s p o n d i n g r a t e o f r e a c t i o n was 9*9x10 molecules /cm / s e c . T h i s r a t e was measured over a 150 Κ temperature range and showed an A r r h e n i u s type dependence y i e l d i n g an a c t i v a t i o n energy o f 19.4 k c a l / m o l e . T a b l e I shows t h e e f f e c t s on t h e r a t e o f gross v a r i a t i o n s i n t h e r e a c t a n t p r e s s u r e s and i n t h e t o t a l p r e s s u r e . The e x t e n s i v e data necessary f o r t h e d e r i v a t i o n o f a t r u e k i n e t i c r a t e law was not c o l l e c t e d . I t c a n be seen c l e a r l y t h a t t h e r e a c t i o n r a t e i s p o s i t i v e o r d e r i n both hydrogen and n i t r o g e n . 2

Table I . E f f e c t of Reactant Pressure on NH S y n t h e s i s (T=748K)

P(N ) (atm)

P(H ) (atm) 2

15 5 5 5

5 15 5 5

P(Ar) (atm)

0 0 10 0

Rate (molec./cm /sec) 2

15

xio]* Ί

18 χ10 £ 4.5x10]ί 7.0x10 D

The s u r f a c e a f t e r t h e r e a c t i o n was found, u s i n g AES, t o be c l e a n w i t h t h e e x c e p t i o n o f the presence o f s t r o n g l y adsorbed n i t r o g e n and some s m a l l amounts o f s u l f u r t h a t segregated from t h e c r y s t a l b u l k d u r i n g t h e course o f t h e r e a c t i o n . Exposure o f t h i s s u r f a c e t o r e a c t i o n c o n d i t i o n s r e s u l t e d i n a r e a c t i o n r a t e s i m i l a r to t h a t o v e r t h e c l e a n s u r f a c e . A l a y e r o f FeN, was prepared by exposing the s u r f a c e t o 5x1 θ " T o r r o f ammonia a t 673 Κ f o r 10 h r s . ( 5 ) . T h i s s u r f a c e had an Auger spectrum s i m i l a r t o t h a t o f the s u r f a c e produced d u r i n g t h e r e a c t i o n and e x h i b i t e d a c a t a l y t i c a c t i v i t y s i m i l a r t o t h a t o f t h e c l e a n s u r f a c e . T h i s suggests t h a t d u r i n g t h e r e a c t i o n t h e amount o f n i t r o g e n d e p o s i t e d on t h e s u r f a c e i s c l o s e t o t h a t p r e s e n t i n t h e compound FeN and p o s s i b l y t h a t t h i s i s a c t u a l l y present at the surface during the r e a c t i o n ( 6 ) . 4

S t r u c t u r e S e n s i t i v i t y o v e r Fe. T a b l e I I p r e s e n t s t h e r a t e s o f ammonia s y n t h e s i s over each o f t h e low M i l l e r index planes o f Fe. The r e l a t i v e r a t e s a r e 418 : 25 : 1 f o r t h e ordered F e ( l 1 l ) , Fe(lOO)


14.

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AL.

157


and P e ( l 1 0 ) s u r f a c e s r e s p e c t i v e l y . These r a t e s are c o n s i s t e n t w i t h the c h e m i s o r p t i o n s t u d i e s of E r t l (3) w h i c h showed t h a t the r a t e of d i s s o c i a t i v e n i t r o g e n c h e m i s o r p t i o n under UHV c o n d i t i o n s f o l l o w s the same p a t t e r n o f s t r u c t u r e s e n s i t i v i t y . T h i s agreement between r e a c t i o n r a t e s and the c h e m i s o r p t i o n s t u d i e s a l s o i n d i c a t e s t h a t i t i s the d i s s o c i a t i v e c h e m i s o r p t i o n s t e p t h a t i s r a t e l i m i t i n g i n the o v e r a l l r e a c t i o n pathway. Table I I .

NH^

S y n t h e s i s S t r u c t u r e S e n s i t i v i t y on


Surface

Rate

Fe(m) F e ( l O O ) , Ordered F e ( l 1 0 ) , Ordered Fe(lOO), Disordered, F e ( l 1 0 ) , Disordered ]

&

(molec./cm

27 1.7 0.1 2.7 0.6 χ

a. P ( H ) - 15 atm., P ( N J - 5 atm. b. Argon i o n bombardment ( 1 . 5 x 10 2

2

Fe

/sec)

15 χ 10 15 χ 10. X 10 15

15

10

5

Τ - 7^8 Κ A/cm , 600

sec,

500

V)

The F e ( m ) s u r f a c e i s composed o f f o u r - f o l d c o o r d i n a t e d atoms and exposed second l a y e r atoms t h a t a r e s e v e n - f o l d c o o r d i n a t e d w h i l e t h e F e d 10) and Fe(lOO) p l a n e s have o n l y s i x - f o l d and f o u r - f o l d c o o r d i n a t e d atoms r e s p e c t i v e l y . Here we have d e f i n e d the c o o r d i n a t i o n number as the number of n e a r e s t n e i g h b o r atoms. Dumesic's p r o p o s a l t h a t the s e v e n - f o l d c o o r d i n a t e d atom i s an i m p o r t a n t component of the c a t a l y t i c a l l y a c t i v e s i t e i s not c o n t r a d i c t e d by our r e s u l t s (4). I t i s a l s o worth n o t i n g t h a t the r e l a t i v e roughness or openess of the each p l a n e f o l l o w s the same p r o g r e s s i o n as t h e i r c a t a l y t i c a c t i v i t i e s . T a b l e I I a l s o shows t h a t t h e a c t i v i t y of each of the two l e s s a c t i v e s u r f a c e s was markedly enhanced by s p u t t e r i n g w i t h A r . I t i s p o s s i b l e t h a t s p u t t e r i n g has exposed s e v e n - f o l d c o o r d i n a t e atoms at the s u r f a c e or t h a t i t i s the roughness of the s u r f a c e t h a t i s r e s p o n s i b l e f o r the s t r u c t u r e s e n s t i v i t y of the r e a c t i o n r a t e . K i n e t i c s o f Ammonia S y n t h e s i s on Re C r y s t a l S u r f a c e s . The k i n e t i c s o f the ammonia s y n t h e s i s r e a c t i o n on Re were s t u d i e d on the R e ( l 1 2 0 ) s u r f a c e which i s composed of atoms h a v i n g s e v e n - f o l d c o o r d i n a t i o n . The r a t e of p r o d u c t i o n a t ψΟ Κ and 20 atm. Η /N i n a 3:1 r a t i o i s 1.8x10 molec/cm / s e c . Over l o n g p e r i o d s of time the r a t e of p r o d u c t i o n decreased due to p r o d u c t p o i s o n i n g o f the s u r f a c e as the c o n c e n t r a t i o n of ammonia i n the r e a c t o r l o o p i n c r e a s e d , an e f f e c t t h a t was not observed on Fe ( 7 ) · An A r r h e n i u s p l o t of the r e a c t i o n r a t e y i e l d s an a c t i v a t i o n energy of 20.0 k c a l / m o l e , v e r y c l o s e to t h a t observed on Fe s u g g e s t i n g t h a t the r a t e d e t e r m i n i n g s t e p i s i d e n t i c a l on the two s u r f a c e s . The dependence of the r a t e on the r e a c t a n t p r e s s u r e s has been s t u d i e d i n g r e a t e r d e t a i l on Re t h a n on Fe and has been f i t t e d t o a r a t e law of the form (8) 2


158


S t r u c t u r e S e n s i t i v i t y o v e r Re. As i n the case of the Fe c a t a l y s t s the r a t e of ammonia s y n t h e s i s v a r i e s g r e a t l y over Re s i n g l e c r y s t a l s u r f a c e s of d i f f e r e n t o r i e n t a t i o n s . _ T h i s phenomenon has been s t u d i e d over the ( 0 0 0 1 ) , (ΐθΤθ), (1120) and (1121) p l a n e s i n a 3:1 H /N m i x t u r e a t a t o t a l p r e s s u r e of 20 atm. and a temperature o r 87Ο K. Under these c o n d i t i o n s these s u r f a c e s c a t a l y z e the r e a c t i o n w i t h r e l a t i v e r a t e s of 1:94:920:2820 r e s p e c t i v e l y , showing a range of a c t i v i t i e s even g r e a t e r than t h a t observed on Fe.


p

Re has a hep c r y s t a l s t r u c t u r e and so the ( 0 0 0 1 ) , the l e a s t a c t i v e , i s a c l o s e packed s u r f a c e w h i l e the (ΐθΤθ) i s somewhat more open and i s composed of t r o u g h s . The most a c t i v e p l a n e s s t u d i e d , the R e ( l 1 2 0 ) , R e d 121) and the F e ( l 1 l ) s u r f a c e s are shown i n F i g u r e 1 w h i c h shows t h a t the most common f e a t u r e ^ among them i s t h e i r open o r rough topography. The F e ( l 1 l ) and R e d 120) s u r f a c e s both have l a r g e numbers of exposed s e v e n - f o l d c o o r d i n a t e atoms, however, the R e d 121), the most a c t i v e of a l l s u r f a c e s , has none. T h i s tends to c o n t r a d i c t the s u g g e s t i o n made p r e v i o u s l y t h a t i t i s t h e s e s e v e n - f o l d c o o r d i n a t e atoms a t the s u r f a c e t h a t are p a r t i c u l a r l y a c t i v e catalytic sites. What seems to be a more g e n e r a l t r e n d i s t h a t i t i s the h i g h c o o r d i n a t i o n of the h o l l o w o r v a c a n t s i t e s at the s u r f a c e t h a t determines the c a t a l y t i c a c t i v i t y . I t i s presumably these h o l l o w s i n t o which a d s o r b s and d i s s o c i a t e s . The

H y d r o d e s u l f u r i z a t i o n o f Thiophene

K i n e t i c s o v e r the Mod 00) C r y s t a l S u r f a c e . We have s t u d i e d the h y d r o d e s u l f u r i z a t i o n of t h i o p h e n e over the i n i t i a l l y c l e a n MO(100) s i n g l e c r y s t a l s u r f a c e i n the t e m p e r a t u r e range 520K - 690K and at r e a c t a n t p r e s s u r e s of 100 T o r r £ P(H ) _< 800 T o r r and 0.1 T o r r _< P(Th) j( 10 T o r r . Under these c o n d i t i o n s the r e a c t i o n i s c a t a l y z e d a t a c o n s t a n t r a t e f o r a p e r i o d o f a p p r o x i m a t e l y one hour a f t e r w h i c h the r a t e b e g i n s to d e c r e a s e w i t h time. The r a t e s r e p o r t e d h e r e are a l l i n i t i a l r a t e s of r e a c t i o n c a l c u l a t e d from d a t a c o l l e c t e d i n the p e r i o d o v e r which they remain c o n s t a n t . The p r o d u c t d i s t r i b u t i o n of the HDS o f thiophene over the MO(100) s u r f a c e i s shown i n T a b l e I I I compared w i t h t h a t r e p o r t e d by K o l b o e o v e r a MoS c a t a l y s t (14). I t i s c l e a r t h a t the two are v e r y s i m i l a r and t h a t our c a t a l y s t mimics the MoS c a t a l y s t v e r y made i n the c l o s e l y i n t h i s respect. t e m p e r a t u r e r e g i o n mentioned above shows t h a t b u t a d i e n e i s the o n l y p r o d u c t whose r a t e of f o r m a t i o n shows t r u e A r r h e n i u s type dependence and y i e l d s an a c t i v a t i o n energy o f 14.4 k c a l / m o l e . At h i g h t e m p e r a t u r e s the r a t e of butane f o r m a t i o n d e v i a t e s even more s h a r p l y than t h a t of the butènes and does so at l o w e r temperatures Q

(9).


G E L L M A N ET A L .


Top


Side View

View

Re(ll 21)

Re (1120)

C

7

F i g u r e 1. The most a c t i v e Fe and Re s u r f a c e s f o r ammonia synthesis.



In (rote) vs.


τ

1

1

I/T 1

Γ

F i g u r e 2 . A r r h e n i u s p l o t f o r the HDS o f thiophene on the MO(100) s u r f a c e . P ( H ) = 780 T o r r , P(Th) = 2 . 5 T o r r . 2


14.

G E L L M A N E T AL.


Table I I I .

P r o d u c t D i s t r i b u t i o n o f Thiophene

MO(1 0 0 )

Product


Butane 1-Butene cis-2-Butene Τrans-2-Butene Butadiene

15.6* 47.5* 14.7* 18.7* 4.1*

a

MoS

161

HDS

b 2

11.5* 41.7* 17.8* 22.8* 6.3*

a. P(H )=780 T o r r , P(Th)=2.5 T o r r , Τ=285°0 b. P(H^)=760 T o r r , P(Th)=20 T o r r , T=288 C

The r e s u l t s o f v a r i a t i o n s i n r e a c t a n t p r e s s u r e s a r e shown i n T a b l e IV. A g a i n t h e r e a r e c l e a r v a r i a t i o n s among each o f the p r o d u c t s p o i n t i n g t o o r d e r s i n hydrogen p r e s s u r e dependence o f z e r o , o n e - h a l f , and one f o r b u t a d i e n e , the butènes and butane r e s p e c t i v e l y . The h a l f - o r d e r dependence o f the butene r a t e o f f o r m a t i o n shows t h a t t h e hydrogénation must be a r e a c t i o n w i t h atomic hydrogen on the s u r f a c e and t h a t the hydrogen c o n c e n t r a t i o n on the s u r f a c e must be i n t h e low coverage regime o f the a d s o r p t i o n i s o t h e r m . These r e s u l t s a l s o p o i n t t o the temperature dependence b e i n g a r e s u l t of a decrease i n the hydrogen coverage a t h i g h t e m p e r a t u r e s . The r a t e o f b u t a d i e n e f o r m a t i o n , b e i n g independent o f H p r e s s u r e , w i l l not be a f f e c t e d by changes i n hydrogen coverage and t h u s , w i l l show A r r h e n i u s type temperature dependence. The r a t e s o f butene and butane f o r m a t i o n w i l l be a f f e c t e d by d e c r e a s e s i n hydrogen coverage a t h i g h t e m p e r a t u r e s . The butane f o r m a t i o n r a t e s h o u l d be more s e v e r e l y a f f e c t e d than t h a t o f the butènes, as i s observed.

T a b l e IV.

HDS

Dependence on R e a c t a n t P r e s s u r e s (Τ=340 C)

Product

Order of P r e s s u r e Dependence ( H )

Order o f P r e s s u r e Dependence (Th)

Butane 1 -Butene cis-2-Butene trans-2-Butene Butadiene

1.10 0.54 0.58 0.68 - 0.18

0.13 0.33 0.16 0.18 0.95

2




162

E f f e c t s o f S u l f u r Coverage* The k i n e t i c r e s u l t s r e p o r t e d i n the p r e v i o u s s e c t i o n are from r e a c t i o n s performed on s u r f a c e s t h a t a r e i n i t i a l l y c l e a n . The s u r f a c e s a f t e r r e a c t i o n were examined by AES and shown to be covered w i t h carbon and s u l f u r at coverages c l o s e t o a monolayer. I t i s i n t e r e s t i n g to note t h a t a l t h o u g h t h i s i s the case even a f t e r r e a c t i o n times on the o r d e r of minutes the r e a c t i o n r a t e i s c o n s t a n t f o r a p e r i o d o f a p p r o x i m a t e l y one hour. E i t h e r the r e a c t i o n i s o c c u r r i n g on top of t h i s c a r b o n / s u l f u r l a y e r o r these s p e c i e s are p r e s e n t as s u l f u r c o n t a i n i n g hydrocarbon fragments t h a t are i n t e r m e d i a t e s i n the d e s u l f u r i z a t i o n p r o c e s s . F i g u r e 3 shows the e f f e c t on the o v e r a l l r e a c t i o n r a t e of s u l f u r adsorbed onto the s u r f a c e under UHV c o n d i t i o n s . The nature of these s u l f u r o v e r l a y e r s has been s t u d i e d i n some d e t a i l and d e s c r i b e d e l s e w h e r e ( 1 0 . 1 1 ) . At coverages of l e s s than one h a l f of one monolayer the s u l f u r has been shown to occupy the f o u r - f o l d h o l l o w a d s o r p t i o n s i t e on the s u r f a c e . I t i s c l e a r t h a t the r a t e of r e a c t i o n decreases as the s u l f u r coverage i n c r e a s e s from 0 . 0 t o 0.5 monolayers and at t h i s p o i n t the f u r t h e r a d d i t i o n of s u l f u r has no apparent e f f e c t . There are two i n t e r e s t i n g p o i n t s about t h i s r e s u l t . F i r s t , i t i s s u r p r i s i n g t h a t the a d d i t i o n of s u l f u r to the s u r f a c e has an e f f e c t on the r e a c t i o n r a t e . I f s u l f u r d e p o s i t e d on the s u r f a c e d u r i n g the HDS o f t h i o p h e n e were i d e n t i c a l i n nature to t h a t d e p o s i t e d b e f o r e the r e a c t i o n , t h e n i n i t i a l s u l f i d i n g o f the s u r f a c e s h o u l d have no e f f e c t on the r e a c t i o n r a t e whatsoever. The observed e f f e c t suggests t h a t the r e a c t i o n o c c u r s v i a a mechanism t h a t e i t h e r does not d e p o s i t s u l f u r onto the s u r f a c e , as has been suggested by o t h e r i n v e s t i g a t o r s (12), o r one t h a t d e p o s i t s s u l f u r i n a form t h a t i s weakly bound and e a s i l y reduced to H S. The second p o i n t o f i n t e r e s t i s t h a t at coverages g r e a t e r than ~ 0 . 5 monolayers the f u r t h e r a d d i t i o n o f s u l f u r to the s u r f a c e does not a f f e c t the r e a c t i o n r a t e . T h i s r e s u l t lends i t s e l f to p o s s i b l e e x p l a n a t i o n i n terms o f two o f the s u l f u r b i n d i n g s i t e s on the s u r f a c e . At h i g h s u l f u r coverages i t has been suggested t h a t s u l f u r o c c u p i e s both the f o u r - f o l d h o l l o w , o c c u p i e d at low coverages, and some o t h e r , p o s s i b l y a b r i d g e d s i t e (10). U l t r a - v i o l e t photo e l e c t r o n s p e c t r o s c o p y s t u d i e s a l s o support the d u a l b i n d i n g s i t e model ( j _ 3 ) . I f the s u l f u r bound to the s u r f a c e i n t h i s h i g h coverage s i t e i s e a s i l y removed from the s u r f a c e by hydrogen, then i t s p r e s e n c e would have no e f f e c t on the r e a c t i o n r a t e . Furthermore, i t i s p o s s i b l e t h a t i t i s t h i s s i t e i n t o which s u l f u r from the r e a c t i o n i s a c t u a l l y deposited. p

Conclusions The s t u d i e s of ammonia s y n t h e s i s over Fe and Re and the hydrode s u l f u r i z a t i o n of thiophene over Mo, d e s c r i b e d above, i l l u s t r a t e the i m p o r t a n c e and success of our approach o f s t u d y i n g c a t a l y s i s over s i n g l e c r y s t a l samples at h i g h p r e s s u r e s . The use of s u r f a c e s h a v i n g a v a r i e t y o f o r i e n t a t i o n s a l l o w s the study o f r e a c t i o n s t h a t are s u r f a c e s t r u c t u r e s e n s i t i v e and p r o v i d e s i n s i g h t i n t o the nature o f the c a t a l y t i c s i t e . Here we have shown t h a t the ammonia s y n t h e s i s



GELLMAN ET AL.


I

ι

ι

ι

ι

f

0

.2

A

.6

.0

1.0

F i g u r e 3· The r a t e o f thiophene HDS on Mo(lOO) v s . P ( H ) = 780 T o r r , P(Th) = 2.5 T o r r , T- 340 C. 2


164


r e a c t i o n i s h i g h l y s e n s i t i v e t o the s u r f a c e s t r u c t u r e and i s b e s t c a t a l y z e d by s u r f a c e s o f a rough n a t u r e h a v i n g v a c a n t o r h o l l o w s i t e s o f h i g h c o o r d i n a t i o n . The a b i l i t y t o p r e p a r e s u r f a c e s t h a t are w e l l c h a r a c t e r i z e d i n terms o f both c o m p o s i t i o n and s t r u c t u r e a l l o w s t h e e f f e c t s o f s u r f a c e contaminants o r dopants t o be s t u d i e d i n d e t a i l . The presence o f "0.5 monolayers o f s u l f u r on the Mo(lOO) s u r f a c e d e c r e a s e s the r a t e o f thiophene h y d r o d e s u l f u r i z a t i o n t o about one h a l f t h a t o f the c l e a n s u r f a c e . F u r t h e r a d d i t i o n o f s u l f u r does n o t e f f e c t the d e s u l f u r i z a t i o n r a t e . We propose t h a t t h i s may be due t o the a d s o r p t i o n o f the a d d i t i o n a l s u l f u r onto weakly bound s i t e s from w h i c h i t i s e a s i l y reduced.


Acknowledgments We would l i k e t o thank the D i v i s i o n o f M a t e r i a l s S c i e n c e s and O f f i c e o f B a s i c Energy S c i e n c e s o f the U.S. Department o f Energy f o r s u p p o r t under C o n t r a c t No. DE-AC03-76SF00098, and both the Exxon R e s e a r c h and E n g i n e e r i n g Co. and the Dow C h e m i c a l Co. Literature Cited 1.

Blakely, D. W.; Kozak, E.; Sexton, Β. Α.; Somorjai, G. A. J . Vac. S c i . & Tech. 1976, 13(5), 1091. 2. Emmett, P. H. "The Physical Basis for Heterogeneous Catalysis"; Plenum: New York, 1975; p. 3. 3. E r t l , G. Catal. Rev. S c i . Eng. 1980, 21, 201. 4. Dumesic, J . Α.; Topsoe, H.; Boudart, M. J . Catal. 1975, 37, 513. 5. Brill, R.; Richter, E. L.; Ruch R. Angew. Chem. Int. Ed. Engl. 1967, 6, 882. 6. Spencer, N. D.; Schoonmaker, R. C.; Somorjai, G. A. J . Catal. 1982, 74, 129. 7. Spencer, N. D.; Somorjai, G. A. J . Catal. 1982, 78, 142. 8. Asscher, M.; Carrazza, J.; Khan, M. M.; Lewis, Κ. B.; Somorjai, G. A. i n press. 9. Gellman, A. J.; Farias, M. H.; Somorjai, G. A. submitted to J . Catal. 10. Clarke, L. J . Surf. S c i . 1981, 102, 331. 11. Farias, M. H.; Gellman, A. J.; Somorjai, G. A. Surf. S c i . 1984, 140, 181. 12. Micovsky, R. J.; S i l v e s t r i , A. J.; Heinemann, H. J . Catal. 1974, 34, 324. 13. Gellman, A. J.; Tysoe, W. T.; Somorjai, G. A. in press. 14. Kolboe, S. Can. J . of Chem. 1969, 47, 352. R E C E I V E D December 6, 1984


Studies of the Kinetics and Mechanisms of Ammonia Synthesis and

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