Selective Epoxidation of Ethylene Catalyzed by Silver - American

19 Selective Epoxidation of Ethylene Catalyzed by Silver Mechanistic Details Revealed by Single-Crystal Studies Charles T. Campbell Downloaded by FUDAN UNIV on February 24, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch019

Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545

The selective epoxide

oxidation

of

ethylene

to ethylene

(C H + 1/2 O -> C H O) 2

4

2

2

4

over Ag is a simple prototype for the entire class of kinetically-controlled, selective oxidation reactions. We have studied the steady-state kinetics and selec tivity of this reaction on clean, well-characterized single-crystal surfaces of silver by using a special apparatus which allows rapid (~20 s) transfer between a high-pressure catalytic microreactor and an ultra -high vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These single-crystal studies have provided considerable new insight into: the reaction pathway through molecularly adsorbed O and C H , the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro moting catalyst selectivity via an ensemble effect. 2

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4

The c a t a l y t i c oxidation of ethylene to ethylene epoxide (also known as ethylene oxide) 0

i s a s e v e r a l - b i l l i o n - d o l l a r per year industry, providing the neces sary intermediate i n the synthesis of ethylene g l y c o l , which i s used i n polyester and antifreeze production. The most common c a t a l y s t i s s i l v e r supported on α - A 1 0 , of ~1 m · g " s p e c i f i c surface area, promoted f o r s e l e c t i v i t y by adding trace amounts of chlorinated hydrocarbons t o the feed. This reaction i s the simplest example of 2

2

1

3

0097-6156/85/0288-0210$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

CAMPBELL

211

Selective Epoxidation of Ethylene

a k i n e t i c a l l y - c o n t r o l l e d , s e l e c t i v e heterogeneous c a t a l y t i c reac tion. (Metals other than s i l v e r generally produce C0 + H 0, the thermodynamically preferred product.) As such, t h i s reaction i s of fundamental importance from the surface science point-of-view and has therefore been the subject of numerous reaction studies (1-21), as w e l l as u l t r a - h i g h vacuum (UHV) adsorption studies [(22-23) and references t h e r e i n ] . However, most of the basic questions con cerning the reaction mechanism and the nature of the a c t i v e c a t a l y s t surface remain unanswered. This i s l a r g e l y due to the fact that the reaction proceeds too slowly to be observable under the vacuum con d i t i o n s t y p i c a l l y employed i n surface a n a l y s i s . This paper reports some of our recent results i n which we have studied the reaction on Ag(llO) and (111) by d i r e c t l y combining high-pressure k i n e t i c meas urements with UHV surface analysis i n a single apparatus (24-28). These studies have provided new insights into the reaction mech anism, i t s s t r u c t u r a l s e n s i t i v i t y , and the role of chlorine i n pro moting s e l e c t i v i t y .

Downloaded by FUDAN UNIV on February 24, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch019

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Experimental D e t a i l s of the apparatus and techniques are presented i n related papers (23,25,28). In short, the Ag(llO) and (111) samples were cleaned by sputtering and annealing (800 K) i n u l t r a - h i g h vacuum (UHV). Cleanliness and order were proven by Auger electron spec troscopy (AES) and low-energy electron d i f f r a c t i o n (LEED). The clean sample was transferred into an evacuated microreactor attached d i r e c t l y to the UHV chamber, pressurized with the reaction mixture (~200 t o r r ) and heated (440-610 K) u n t i l a steady-steady epoxidation rate (2-4 min) was established. [Steady-state rates could be main tained over times during which >5000 molecules of ethylene per sur face Ag atom were converted into product (25,28).] Then the sample was r a p i d l y (17 s) transferred at reaction temperature back into UHV for surface analysis (AES, LEED, TDS, XPS). The coverage of 0 was q u a n t i t a t i v e l y measured by f l a s h heating the sample immediately a f t e r t r a n s f e r and measuring the area under the 600 Κ 0 thermal desorption peak mass-spectrometrically (25). A f t e r t r a n s f e r , the reaction mixture ( s t i l l i n the batch microreactor) was analyzed by gas chromatography f o r the amount (rates) of ethylene oxide and C0 produced. As discussed i n related papers (25,28), above 480 Κ the surface contained only atomically adsorbed oxygen and maintained a good LEED pattern a f t e r reaction. Below 480 Κ other adsorbed reactants and products were observed. The sides and back of the c r y s t a l were passivated to reaction by a mixed S i , Cu, T i oxide/carbide f i l m that b u i l t up during the e a r l y stages of high-pressure reaction attempts. 2

2

Coverages (Θ) are defined r e l a t i v e to the number of Ag surface atoms [Θ = 1 i s 8.5 x 1 0 cm" f o r Ag(110) and 1.38 x 1 0 cm" f o r Ag(lll)]. Sample sizes were: Ag(110) = 10 i n x 6 mm x 2 m and A g ( l H ) = 10 mm x 7 urn x 2 mm. 14

2

15

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The methods f o r depositing chlorine adatoms on the (110) sur face have been described previously (26), and resulted i n surface structures and LEED patterns i d e n t i c a l to those achieved by d i s s o c i a t i v e C l adsorption. A c(4x2)-Cl pattern at θ^,, = 0.75 (26) was used to c a l i b r a t e chlorine coverages, which were taken propor t i o n a l to the r a t i o of Cl:Ag AES i n t e n s i t i e s (26). 2

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

212

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

Results and Diseus s ion Structural Sensitivity. Figure 1 shows the steady-state rates of ethylene oxide (EtO) and C0 production as a function of temperature, i n Arrhenius form, at an ethylene pressure (P-,. ) of 20 t o r r and Ρ r*t Ij2 = 150 t o r r on clean Ag(110) (24,25). These s p e c i f i c rates i n F i g . 1 are expressed i n terms of turn-over-number (TON), i . e . , the number of molecules of each produced per Ag surface atom ( s i t e ) per second, assuming 1 0 Ag surface atoms on our sample. Shown f o r comparison i s the s p e c i f i c rate of EtO production over a high-surface-area, silica-supported Ag c a t a l y s t under the same conditions. The a c t i v a t i o n energy f o r EtO production decreases from about 22.4 to 5.3 kcal mole" as the temperature increases from 440 to 610 Κ on Ag(110), almost i d e n t i c a l to the behavior on the supported Ag c a t a l y s t . 2

Λ

15


1

The s e l e c t i v i t y f o r ethylene conversion i n t o EtO ( S [T0N

Et()

£ t 0

=

T 0 N E t

Q/

+ 1/2 T O N ^ ] ) from the data of F i g . 1 f o r Ag(110) shows a

gradual decrease with temperature (24), i n agreement with most highsurface-area catalysts (15,17,29). The absolute value of the selec t i v i t y (~40%) on Ag(110) f a l l s w i t h i n the range seen f o r unpromoted, high-surface-area Ag catalysts (28). We have found (24,25) that the e f f e c t s of temperature and reactant pressures upon the rates of EtO and C0 production and upon s e l e c t i v i t y are v i r t u a l l y i d e n t i c a l on Ag(110) and high-surface-area Ag c a t a l y s t s . However, the absolute s p e c i f i c rates (per surface Ag atom) f o r the (110) surface were found to be some 100-fold higher than those reported f o r a v a r i e t y of high-surface-area catalysts (24). We had postulated that a small percentage of (110) planes or (110)-like s i t e s are responsible f o r most of the c a t a l y t i c a c t i v i t y of high-surface-area catalysts (24,25). However, our very recent r e s u l t s f o r the clean, close-packed A g ( l l l ) face (28) show that i t s s p e c i f i c a c t i v i t y i s only a factor of about two below the (110) face. Since the lowest-energy (111) face i s expected to predominate on high-area c a t a l y s t s , the huge difference i n s p e c i f i c a c t i v i t i e s for s i n g l e - c r y s t a l Ag and high-area catalysts seems no longer to be due to some inactive Ag surface predominating the high-area cat a l y s t s . We note that the k i n e t i c s ( a c t i v a t i o n energies and reaction orders) on A g ( l l l ) and (110) are very s i m i l a r (28). 2

This reaction has long been known to be c l a s s i c a l l y structur a l l y s e n s i t i v e ; i . e . , the rates and s e l e c t i v i t y depend s e n s i t i v e l y upon the s i z e and shape of Ag p a r t i c l e s i n high-surface-area cat a l y s t s (10,14,24,30,31). The rate v a r i a t i o n s apparently cannot be related to v a r i a t i o n s i n the concentration of active surface facets with p a r t i c l e s i z e . I t may be that some impurity from the prepara t i o n process or support material poisons the c a t a l y s t surface, and the concentration of t h i s poison i s dependant upon the same factors i n the preparation scheme which allow f o r p a r t i c l e size adjustment. Three further facts might support t h i s explanation: (1) A strange minimum i n s p e c i f i c a c t i v i t y with p a r t i c l e size i s observed at the s u r p r i s i n g l y large size of 500-700 Â (10,14,30); (2) the v a r i a t i o n s i n s p e c i f i c a c t i v i t y with p a r t i c l e s i z e observed by any given author (10,14,30) are smaller than the v a r i a t i o n s among authors f o r cata l y s t s of nominally the same p a r t i c l e size [see Table I i n (24) ] ; Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


CAMPBELL

Selective Epoxidati . of Ethylene

Figure 1. The steady state rates of EtO and CO2 production as a function of temperature, i n Arrhenius form, at P = 20 t o r r and PQ2 = 150 t o r r on clean A g C l l O ) . E t


CATALYST CHARACTERIZATION SCIENCE

214

and, (3) the s p e c i f i c a c t i v i t i e s of unsupported, high-area Ag cat a l y s t s are at the upper l i m i t of a c t i v i t i e s seen f o r high-area cat a l y s t s , and only a factor of about 10 below those f o r Ag(110) or (111) [see Table I i n (24)]. For small p a r t i c l e s , s e l e c t i v i t y increases with p a r t i c l e s i z e (14,30). In our r e s u l t s , A g ( l l l ) has an average s e l e c t i v i t y which i s about 6% higher ( i n absolute units) than Ag(110) (28). Since the p a r t i c l e surface should evolve from open, or ( H O ) - l i k e , to closepacked, or ( l l l ) - l i k e , with increasing s i z e , these results may be favorably correlated.


Reaction K i n e t i c s and Mechanism Figure 2 shows the e f f e c t of ethylene pressure upon the steady-state reaction rates f o r clean Ag(110) at 490 Κ and 150 t o r r 0 (25). Also shown i s the steady-state coverage of atomically adsorbed oxygen (0 ) under reaction conditions (measured as described above). The coverage of atomic oxygen decreases with increasing P^, but main tains a s i g n i f i c a n t value (Θ = 0.1) when the rates saturate i n ethylene. 2

q

0

We have measured data s i m i l a r to F i g . 2 where, instead, the e f f e c t s of 0 pressure upon the steady-state rates and θ were ob served (25). These data are p l o t t e d i n F i g . 3 to directîy r e f l e c t the e f f e c t s of oxygen a da torn coverage upon the rates at a f i x e d temperature and ethylene pressure. While the order i n Ρ varies υ strongly with P^ , EtO production remains almost f i r s t order i n throughout the e n t i r e range of 0 pressures. This c l e a r l y indicates that adsorbed oxygen adatoms play an important role i n the reaction mechanism, such that t h e i r coverage enters the rate equation i n a l i n e a r fashion. 2

Λ

2

2

S i m i l a r data on A g ( l l l ) i n d i c a t e that under i d e n t i c a l reaction conditions, the atomic oxygen coverage on A g ( l l l ) i s a f a c t o r of about 15 below θ on Ag(110), on the average (28). This can be d i r e c t l y ascribed to the much higher d i s s o c i a t i v e s t i c k i n g p r o b a b i l i t y f o r 0 displayed by the (110) face (22-23,28). Since the epoxi dation rates are only a f a c t o r of about two smaller on A g ( l l l ) , t h i s r e s u l t suggests that atomically adsorbed oxygen may not be d i r e c t l y involved i n the epoxidation step. We w i l l show below further data which support molecularly adsorbed (peroxo-) 0 as the true o x i d i zing agent. Since 0 i s a precursor to 0 formation (23), i t s coverage w i l l be proportional to θ at constant temperature and ethylene pressure ( F i g . 3), provided θ i s low and 0 i s removed from the surface only v i a a reaction which i s f i r s t order i n θ (28). ο — I n t e r e s t i n g l y , we have shown by postreaction LEED that those oxygen adatoms reside on the surface under reaction conditions i n p(2xi)-0 islands of l o c a l coverage θ = 0.5 on Ag(110) or p(4x4)-0 islands of l o c a l coverage θ = 0.4 on A g ( l l l ) . This suggests that reaction occurs at the edges of these islands (25-28). 2

2

2

a



0 2

Figure 2. The e f f e c t of ethylene pressure upon the steady s t a t e r e a c t i o n r a t e s f o r c l e a n Ag(100) at 490 Κ and P = 150 t o r r .

r +

2

Figure 3. The e f f e c t of 0 pressure upon the steady s t a t e r e a c t i o n r a t e s f o r clean Ag(110) at 490 Κ and P = 4.1 t o r r .


216

CATALYST C H A R A C T E R I Z A T I O N SCIENCE

Note i n F i g . 3 a d i s t i n c t break i n C0 production rate versus 6. We relate t h i s break to the existence of two pathways to C0 production (25). Under most conditions ( i n t h i s case f o r θ > 0.03), C0 production i s predominated by a mechanism which proceeds through the same rate-determining step as the epoxidation path. Thus e x i s t the very strong k i n e t i c s i m i l a r i t i e s f o r EtO and C0 production (2,3,6,25). The other path to C0 i s less understood (25) and only predominates under extreme conditions, but i s more s e n s i t i v e to θ^. 2

q

2

2

2

2

The data i n Figs. 2 and 3 suggest a reaction which requires a d e l i c a t e balance between adsorbate coverages, consistent with a Langmuir-Hinshelwood mechanism. More extensive data of t h i s type (24-27) indicate that molecularly adsorbed ethylene and 0 are the c r i t i c a l species, consistent with the mechanism proposed below. Downloaded by FUDAN UNIV on February 24, 2017 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch019

2

The Role of Chlorine Promoters Figure 4 shows the e f f e c t s of chlorine adatom coverage (θ^τ) upon the rates, s e l e c t i v i t y and θ at a set of f i x e d pressure and temper ature conditions on AgOlO? (26). The e f f e c t s are q u a l i t a t i v e l y i d e n t i c a l to those seen by adding trace quantities of chlorinated hydrocarbons to the feed i n real-world Ag c a t a l y s t s : the s e l e c t i v i t y to EtO i s promoted a t , however, some loss i n o v e r a l l a c t i v i t y (1-2,8,26,32-36). This confirms that the promoter e f f e c t i s due to d i r e c t CI atom - Ag i n t e r a c t i o n s , and i s unrelated to the support material (e.g. A 1 0 ) . Note that the major e f f e c t occurs f o r chlo rine coverages between the ρ(2X1) structure ( θ ^ = 0.5) and the c(4x2) structure ( θ ^ = 0.75). The high coverage t o r those changes and i t s c o r r e l a t i o n with d i s t i n c t overlayer structures indicates that an ensemble rather than e l e c t r o n i c factor plays the dominant role i n promotion (26). 2

3

Note i n F i g . 4 that the oxygen adatom coverage i s already com p l e t e l y suppressed by = 0.25, at which point the reaction rates have hardly changed. This further suggests that 0 rather than 0 plays the dominant role i n the reaction mechanism. The decay i n 0 with almost p e r f e c t l y r e f l e c t s the dramatic decrease i n the d i s s o c i a t i v e s t i c k i n g p r o b a b i l i t y f o r 0 with chlorine coverage (26,27). We further found that, i n t h i s coverage range, the adsorp t i o n of molecular (peroxo-) 0 i s hardly affected by (27), con s i s t e n t with the minor effects of chlorine on the epoxidation rate for 0 < 0.25 (Fig. 4). 2 > a

q

2

2

C 1

The s o p h i s t i c a t i o n with which one can monitor the effects of a surface modifier i n a c a t a l y t i c reaction i s demonstrated i n F i g . 5. Using a whole series of measurements such as F i g . 4, with a broad range of temperatures and reactant pressures near 563 Κ, Ρ = Λ

150 t o r r , and P„ = 20 t o r r , we were able to determine d i r e c t l y the v a r i a t i o n s with i n the apparent a c t i v a t i o n energies (E^) and reaction orders (m,n) f o r both EtO and C0 production (27). At a given chlorine coverage, the reaction rates are w r i t t e n as: 2

m

TON = ν exp[-E /RT] P ^ P a

n t

.

The r e s u l t s are shown i n F i g . 5. Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

(1)

Selective Epoxidation of Ethylene


CAMPBELL

Figure h. The e f f e c t s of chlorine adatom coverage ( ) upon the r a t e s , s e l e c t i v i t y , and θ on Ag(llO)' at ^90 Κ and P Q = 150 t o r r and P == U.1 t o r r . C l

0

E T


2

218

CATALYST CHARACTERIZATION

SCIENCE

• Ο

ftC l


0.9

UJ Û

Selective Epoxidation of Ethylene Catalyzed by Silver - American

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