Oxidation of ethanol on silica supported noble metal and bimetallic catalysts

Oxidation of ethanol on silica supported noble metal and bimetallic catalysts

Applied Catalysis, 18 (1985) 57--70 57 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands OXIDATION OF ETHANOL ON SILICA SUP...

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Applied Catalysis, 18 (1985) 57--70

57

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

OXIDATION OF ETHANOL ON SILICA SUPPORTED NOBLE METAL AND BIMETALLIC CATALYSTS Richard D. GONZALEZ Department of Chemical Engineering, U n i v e r s i t y of I l l i n o i s Chicago, I l l i n o i s

at Chicago, Box 4348,

60680, U.S.A.

and Masatoshi NAGAIa Department of Chemistry, U n i v e r s i t y of Rhode Island, Kingston, Rhode Island 02852, U.S.A. aon leave from Tokyo University of A g r i c u l t u r e and Technology (Received 15 October 1984, accepted 18 April 1985) ABSTRACT The oxidation of ethanol has been studied over a series of silica-supported noble metal c a t a l y s t s . Pt was found to be the most active c a t a l y s t for the formation of CO2. Ru was found to be highly s e l e c t i v e f o r the formation of acetaldehyde in the 40-I00C temperature range. A low temperature mechanism leading to the formation of acetaldehyde and H20 was found to occur through an adsorbed ethoxy intermediate. In order to explain the high temperature mechanism which appears to favor the d i r e c t oxidation of ethanol to C02 without the formation of an acetaldehyde intermediate, an adsorbed surface acetate is suggested. Both the surface ethoxy species and the adsorbed acetate have been i d e n t i f i e d using i n s i t ~ infrared spectroscopy. The formation of both acetic acid and ethyl acetate at high temperatures is f u r t h e r evidence that a surface acetate species is present. INTRODUCTION Studies on the oxidation of ethanol have, in general, been aimed at developing processes which maximize the formation of p a r t i a l oxidation products such as acetic acid, acetaldehyde and ethyl acetate, while minimizing the formation of undesirable by products such as CO2 and H20. For t h i s reason, studies on the oxidation of ethanol have been carried out under mild o x i d i z i n g conditions over such catalysts as molybdena [ I - 2 ] ,

thorium molybdate [ 3 ] , tantalum oxide [4] and alumina supported Fe, Co,

Ni, Cu, Ag and Au catalysts [5].

In contrast to these p a r t i a l oxidation studies,

investigations which deal with the complete oxidation of ethanol to CO2 and H20 are less numerous. Because the emission of oxygenated hydrocarbons into the environment leads to the formation of photochemical smog, c a t a l y t i c studies which focus on the d e s i r a b i l i t y of converting oxygenates to CO2 and H20 have been receiving renewed a t t e n t i o n . Most worthy is the research c u r r e n t l y in progress at General Motors which has been concerned with the development of c a t a l y t i c converters f o r the control of auto emissions from alcohol fueled vehicles sold in B r a z i l . McCabe and Mitchell [6-7] performed these studies under conditions which approach those found in actual 0166-9834/85/$03.30

© 1985 Elsevier Science Publishers B.V.

58 auto exhaust systems ( i . e . high space v e l o c i t i e s and variable temperatures). Under these conditions, problems associated with d i f f u s i o n and mass t r a n s f e r l i m i t a t i o n s had to be addressed. A d d i t i o n a l l y , ethanol conversions were g e n e r a lly high. These authors [6-7] found Pt/AI203 to be the most e f f e c t i v e catalyst f o r ethanol conversion to CO2 and H20. Hopcalite, (80% Mn02-20% CuO) also showed a high i n i t i a l

activity

but was found to deactivate i r r e v e r s i b l y under reaction conditions [ 7 ] . A p a r a l l e l reaction pathway leading d i r e c t l y to CO2 and H20 or to the same reaction products through an acetaldehyde intermediate was suggested. Because of these results we f e l t that an ethanol oxidation study performed at lower conversions and somewhat lower space v e l o c i t i e s might shed a l i t t l e

more l i g h t

on the mechanism of t h i s rather i n t e r e s t i n g reaction. Under these conditions, i t would be possible to f o l l o w the fate of the oxygenated intermediates and, i f possible determine the conditions under which the formation of acetaldehyde is e i t h e r minimized or shifted to lower reaction temperatures. For t h i s reason we performed an ethanol oxidation study over a series of silica-supported noble metal catalysts and a well characterized Pt-Ru b i m e t a l l i c c l u s t e r c a t a l y s t . The choice of a s i l i c a support was predicted on our desire to minimize dehydration reactions leadin9 to products such as d i e t h y l ether and ethylene which are formed when metals are supported on more a c i d i c oxides such as W/A1203 [ 7 ] , Cu/SiO2-AI203 [ 5 ] , or on the pure oxide at higher temperatures [7-8]. EXPERIMENTAL SECTION Apparatus and procedure The reactor and associated flow system used in t h i s study are shown in Figure I. All parts except the reactor and ethanol saturator were constructed from stainless steel. The reactor was constructed from 12 mm Pyrex glass tubing and had a volume of 1.5 ml. The c a t a l y s t was held in place by means of a f r i t t e d ceramic disk and quartz wool. The reactor, which was placed in a v e r t i c a l p o s i t i o n , was e x t e r n a l l y heated using an oven connected to a v a r i a b l e temperature programmer (Valley Forge Model PC-6000). 0.1 g of c a t a l y s t (approximate volume 0.23 ml) was charged to the reactor f o r a t y p i c a l series of experiments. For the infrared spectral studies a c e l l was used which was capable of being operated e i t h e r as a pulse microreactor or as a single-pass d i f f e r e n t i a l reactor. The c e l l was designed in such a way that reactant gases were forced through a sample disk with l i t t l e

or no leakage around the edges. Details regarding i t s con-

st ructio n have been published elsewhere [ 9 ] . However, modifications in the design of the o r i g i n a l i n f r a r e d c e l l were made in order to reduce the dead space within the c e l l from 40 to 2 ml. Infrared spectra were obtained using a Nicolet 60SX spectrophotometer equipped with a mercury cadmium t e l l u r i d e detector. The data were processed using the associated Nicolet IR-80 data system. To prevent condensation of the reaction gas mixture, the stainless steel tubing, including the s i x - p o r t valves which were in close proximity to the reactor were

59

Sample

Air

Loop

(~) #

i He FIGURE I

Diagram of flow system. ( I ) rotameters, (2) ethanol saturator, (3) 6-port

valve, (4) reactor, (5) 6-port valve, (6) gas chromatograph, (7) bubble meter, (8) needle valve. heated to 170°C using resistance tape. The reaction temperature was measured using a chromel-alumel thermocouple positioned at the center of the catalyst bed. The t o t a l flow rate of the reactant gas feed mixture was f i x e d at 55 ml min -I Ethanol was introduced into the reactor by bubbling a stream of pure nitrogen at a flow rate of approximately 11 ml min - I through an ethanol saturator maintained in an ice bath at O°C. The concentration of ethanol in the feed stream could be varied from 2200 to 2600 ppm by adjusting the nitrogen flow rate through the saturator. The flow rate was maintained at 4 ml min - I .

Sampling was performed by i n j e c t i n g a

I ml sample from a sampling loop into the gas chromatograph. Materials The gases used in t h i s study were subjected to the f o l l o w i n g p u r i f i c a t i o n t r e a t ment: Nitrogen (National Welders I n d & Medical Gases) was passed through a Supelco Oxygen P u r i f i e r to remove traces of 02 . A i r (National Welders Ind. & Medical Gases) was dried by passing i t through a molecular sieve at 25°C. Hydrogen (National Welders Ind. & Medical Gases) was passed through an oxygen p u r i f i e r backed up by an MnO trap. H2 and He were of the highest a v a i l a b l e p u r i t y . The s i l i c a - s u p p o r t e d samples used in t h i s study were prepared by impregnation or coimpregnation. I n i t i a l l y ,

the appropriate weight of RuCI3.3H20, H2PtCI6.6H20,

RhCI3.3H20 or PdCI2.3H20 (Strem Chemical) was dissolved in an amount of deionized water s u f f i c i e n t to ensure the complete wetting of the support. The solutions were mixed with Cab-O-Sil, Grade M-5 (Cabot Corp., Boston MA; Surface area 200 m2 g - 1

60

average pore diameter 14 nm). The r e s u l t i n g s l u r r y was dried in a vacuum desiccator at room temperature f o r I or 2 days and s t i r r e d r e g u l a r l y during the drying period to retain u n i f o r m i t y . Chemisorption measurements were carried out using the dynamic pulse technique [10]. For use in the spectroscopic reactor, the dried c a t a l y s t was ground into a powder, less than 45 ~m, and pressed i n t o s e l f - s u p p o r t i n g disks I cm in diameter -2 with an optical density of approximately 25 mg cm

~

/

20

x

J

PT-RU

c~ ~o

° o

FIGURE 2

20

Z*O 60 Temperature

80 ( "C )

40

}00

d0 8'0 TEMPERATURE{°C)

IOO

Product d i s t r i b u t i o n as a function of temperature over Pt/SiO 2.

O Ethanol, (~ carbon d i o x i d e , O w a t e r , F ~ a c e t a l d e h y d e , ' a c e t i c

acid,

ethylacetate. Catalyst was pretreated in a i r at 400°C f o r 2 h. FIGURE 3

Ethanol conversion as a function of temperature for a series of s i l i c a -

supported noble metal catalysts. O P t ,

( ~ P t - R u , O Ru, F ' I P d , ~ K - R h , A SiO2.

Catalysts were pretreated in a i r at 400°C f o r 2 h. Catalyst pretreatment Pretreatment in He was performed by flowing He over the c a t a l y s t at room temperature (25 ml min - I ) f o r 0.5 h. The temperature was then increased to 400°C at a rate of I0°C min - I . Pretreatment in e i t h e r H2 or a i r was performed by heating the c a t a l y s t (I0°C min - I ) in flowing He to 400°C. The c a r r i e r gas was then switched to e i t h e r H2 or a i r (25 ml min - I ) and treated f o r 2 h. The c a t a l y s t was then cooled to 50°C in flowing He.

61

TABLE Dispersion and metal loadings of catalysts Catalyst

Metal loading /m moles g-1 of SiO2

Dispersion a /%

Pt/Si02 b Pt-Ru/SiO 2

0.3

29

Pt=0.15, Ru=O.15

28

Ru/SiO2

0.2

13

Pd/SiO2

0.3

26

Rh K/SiO 2

Rh=O.3, K=O.03

28

ameasured using CO chemisorption bsurface composition was 75% Pt, 25% Ru. Analysis Product analysis was performed by gas chromatography. A 3 meter stainless steel column (3 mm diameter) packed with chromosorb 106 (60/80 mesh) was found to be adequate for product separation.

The column was operated at 80°C (3.5 min) and -I ramped to 200°C at a rate of 12°C min Reaction products were confirmed by comparing retention times to those of the known pure compounds using both chromosorb 106 and Poropak Q. CO, C2H4, C2H2, C2H6

and C3H4, as reported by Lengendre and Cornet [ 4 ] , were not observed. Methyl acetate and diethyl ether [7] were not detected. RESULTS The reaction product d i s t r i b u t i o n resulting from the steady state oxidation of ethanol over a 5% Pt-SiO 2 catalyst pretreated in a i r at 400°C f o r 2 hours is shown in Figure 2 as a function of temperature. Acetaldehyde formation reached a maximum at 50°C and decreased to zero at about I00°C. Above I00°C, CO2 and H20 were the primary reaction products while only trace amounts of acetic acid, ethyl acetate and acetaldehyde were observed. The products shown on the ordinate in Figure 2 represents the volume in a I ml sample loop injected into the gas chromatograph. The percentage of ethanol converted under steady state conditions for a series of supported

noble metal catalysts as a function of temperature is shown in Figure

3, Metal loadings and dispersions f o r each c a t a l y s t are shown in Table I. The surface composition of the Pt-Ru/SiO 2 b i m e t a l l i c cluster was 74% Pt, as measured by the 02-C0 t i t r a t i o n technique described by Miura and Gonzalez [11]. For the case of Pt/SiO 2 and Pt-Ru/SiO 2 catalysts the temperature dependence of ethanol conversion was s t r i k i n g l y d i f f e r e n t from that observed for the other supportednoble metal catalysts studied. As shown in Figure 3, ethanol conversion increased r a p i d l y with increasing temperature on Pt/SiO 2 and Pt-Ru/SiO 2, but increased only s l i g h t l y on the

62

~o

/



o (J 1.0

.

i

40

.

I

.

60

Temperature

I

80

i

~00

('C)

FIGURE 4 Carbon dioxide formation as a function of temperature for a series of silica-supported noble metal catalysts. 0

Pt, (~ Pt-Ru, O Ru, [ ]

Pd, A Rh-K.

Catalysts were pretreated in air at 400°C for 2 h. Pd/SiO2 and the potassium promoted Rh/SiO2 catalysts. Ethanol conversion on Ru/SiO2 showed no appreciable dependence on temperature. Surprisingly, Cab-O-Sil showed a small amount of a c t i v i t y for the conversion of ethanol to acetaldehyde. A blank run performed in the absence of a catalyst showed no catalytic a c t i v i t y . For this reason we conclude that the stainless steel tubing was inert and additionally, ther, was no gas phase reaction. The steady state rate of CO2 formation over the same series of silica-supported noble metal catalysts is shown in Figure 4. CO2 formation increased rapidly with temperature on both the Pt/SiO2 and Pt-Ru/SiO2 catalysts. However, i t remained remarkably constant on the other noble metal catalysts studied. Similar results wer observed for the rate of water formation (Figure 5A). The steady state rate of acetaldehyde formation was observed to go through a maximum at 50°C on Pt/SiO2. The maximum in the rate of acetaldehyde formation was observed to s h i f t upwards by 20°C on the Pt-Ru/SiO2 catalysts (Figure 5B). The rate of both acetaldehyde and acetic acid formation did not change appreciably with temperature on the Ru/SiO2 catalyst. From these results we conclude that under the conditions of this study only the Pt/SiO2 catalysts showed s i g n i f i c a n t a c t i v i t y for ethanol conversion to CO2 and H20 in the temperature range 40-I00°C. The effect of catalyst pretreatment on the steady state rate of CO2 formation is shown in Table 2. Pt/SiO2 showed a s i g n i f i c a n t improvement in the steady state rate of CO2 formation following pretreatment in either flowing H2 or a i r at 400°C.

20

63

/A//

20 I (B)

x

~

o

1.0

u

4Jo

60I Temperature

i 80I I00 (°C)

I

410

60 Temperature

810 I00 ( °C )

1.0

o/,--Cl~
40

,

60

Temperature

FIGURE 5

~, ,

80 ('C)

100

The formation of water (A), acetaldehyde (B), and acetic acid (C) as a

function of temperature over a series of supported metal c a t a l y s t s . 0 •

Pt, (~ Pt-Ru,

Ru. Pretreatment was in a i r at 400°C f o r 2 h.

Pretreatment in e i t h e r He, H2 or a i r at 400°C had p r a c t i c a l l y no e f f e c t on the rate of CO2 formation on the other supported noble metal catalysts studied. As shown in Table 2, steady state reaction rates of CO2 formation were over an order of magnitude larger on the Pt/SiO 2 and Pt-Ru/SiO 2 catalysts at I00°C than on the other supported noble metal catalysts studied. Effect of reactant concentration The dependence of the reaction rate on the flow rate of a i r is shown in Figure 6A. Because of the large oxygen excess in the reactant gas mixture, the product d i s t r i b u t i o n was not s i g n i f i c a n t l y a l t e r e d by changing the flow rate. The dependence of the rate on the p a r t i a l pressure o f ethanol was studied by changing the contact time. The r e s u l t s shown in Figure 6B demonstrate a p o s i t i v e reaction order of approximately 0.6 in the p a r t i a l pressure of ethanol at 75°C. To i n v e s t i g a t e the oxidation of ethanol in the absence of a i r , a supported Pt-Ru b i m e t a l l i c c a t a l y s t pretreated in a i r at 400°C f o r two hours was cooled to 50°C in flowing a i r . A gas stream consisting of pure N2 and ethanol was then passed over the catalyst at 50°C. The i n i t i a l

rate of ethanol o x i d a t i o n to acetaldehyde and

64

TABLE 2 The e f f e c t of pretreatment on CO2 formation Pretreatment

Reaction temp. /°C

He,400°C, 2 h Air,400°C 2 h H2400°C 2 h

CO2 formation/ml x 104 Pt

Pt-Ru

Ru

Pd

Rh-K

50

1.7

2.4

2.3

-

1.1

100

18.2

12.8

3.3

-

1.1

50

5.0

3.8

2.6

1.5

2.4

100

27.0

12.4

2.6

1.5

2.5

50

6.6

2.6

2.2

1.0

0.8

100

29.0

11.8

2.5

1.6

1.6

water was high, while the formation of CO2 and other oxygenated products was very low. The rate of acetaldehyde formation dropped to zero with the depletion of the oxygen adlayer which was retained by the c a t a l y s t f o l l o w i n g pretreatment in a i r at 400°C (Figure 7). When the rate of ethanol conversion had decreased to a small but measurable amount, oxygen was added to the gaseous reaction feed stream. This resulted in a rapid increase in the rate of ethanol oxidation u n t i l the steady state rate was a t t a i ne d . Following the attainment of a steady state reaction rate, the flow of 02 was again cut o f f . The reaction rate dropped to zero, suggesting that the oxygen bound to the surface f o l l o w i n g pretreatment in a i r at 400°C could sustain the reaction f o r longer periods of time than oxygen chemisorbed at 50°C under reaction conditions. These results suggest the storage of a reasonably large inventory of the chemisorbed and l a t t i c e oxygen f o l l o w i n g pretreatment in a i r at 400°C. Both types of oxygen are apparently capable of sustaining the o x i d a t i o n of ethanol. When the l a t t i c e oxygen is depleted, chemisorbed oxygen can sustain the reaction f o r only a very short time. In situ infrared studies

The infrared spectra of the species present on the surface during the oxidation of ethanol on Pt/SiO 2, Ru/SiO2 and SiO2 at I00°C are shown in Figure 8. Similar in situ

spectra were obtained at 50°C. However, because of extensive ethanol physi-

sorption, the r e s u l t i n g spectra were of l i t t l e

value due to interference of the

physisorbed species in the spectral region of i n t e r e s t . At 100%, only strongly chemisorbed species were observed in the i n f r a r e d spectrum. The three bands centered at 2985, 2938 and 2906 cm-I are due to the symmetric and antisymmetric stretching v i b r a t i o n s of the CH3 and CH2 groups of adsorbed ethyoxy species. The rather poorly developed band structure observed in the 1450 cm-I spectral region is due to the out of plane bending v i b r a t i o n s of the CH3 and CH2 groups. These same features were also observed on Ru/SiO2 and on the s i l i c a support. The intense absorption centered at 1723 cm- I with a high frequency component at 1740 cm- I and a small shoulder at

65 10

T 7sc

(B)

;5

(a)

T: 50"C

1o

- - 0

"~ 0.5 D.

E

g--

i o.s

-(~-

>

O-

__.m

o " f-

A 6 Air

FIGURE 6

Flow

12

8

0

~llL----~e 2

4

W/F

(mllmin)

6

8

( g~cat, rninlml )

Product formation over Pt-Ru/SiO 2 as a function of flow rate of a i r (A)

and contact time ( B ) . O H 2 0 , (~ carbon dioxide, [ ] •



m.

acetaldehyde, •

acetic acid,

ethanol. Pretreatment was in a i r at 400°C f o r 2 h.

t C~OH~. N2: C~H5OH+ N2 + Air A

"~

z.o

_.=

IIC2HsOH~/~•,,,-

/

:

,.o

1

FIGURE 7

2 3 Z, Time (hr)

6

7

Product d i s t r i b u t i o n over a Pt-Ru/SiO 2 catalyst as a function of time in

the presence and absence of a i r . • e t h a n o l , [ - ] dioxide, ~

5

ethyl acetate, •

acetaldehyde, 0

water,(~carbon

acetic acid. Pretreatment was in a i r for 2 h at

400°C. Reaction temperature was 50°C. 1640 cm-I was observed under reaction conditions only on Pt/SiO 2. We assign the intense band at 1722 cm-I to the carbonyl stretching frequency of a surface acetate species. The i n f r a r e d bands at 1740 cm- I and 1640 cm-I were not always present. Assignment of these bands is not c e r t a i n . However, the high frequency band could be due to a dimeric surface species. The infrared band centered at 1640 cm-I is in

66

.13A

o

8

oa

RU/SI 02

~ / ~ O0

2870

2540

2210

1880

1550

I 20

3200

70

2540

(CN'I)

FIGURE 8

2210

1880

50

I 2C

(CM-I)

In situ i n f r a r e d spectra obtained during the ox id a t io n of ethanol on

Pt/SiO 2, Ru/SiO2 and SiO2 at I00°C. FIGURE 9 In situ infrared spectra obtained over Pt/SiO 2, Ru/SiO2 and SiO2 f o l l o w i n reduction of the p a r t i a l pressure of oxygen to zero at I00°C. a l l l i k e l i h o o d due to surface carbonate. The band centered at 2051 cm-I is due to chemisorbed CO which is formed e i t h e r by the d i s s o c i a t i v e adsorption of CO2 or as a r e s u l t of the decomposition of the surface acetate species. When the flow of a i r was discontinued, the infrared band due to chemisorbed CO -I was observed to i n t e n s i f y at the expense of the sharp band centered at 1725 cm (Figure 9). In a d d i t i o n , the high frequency component at 1740 cm-I was not observed suggesting that perhaps the CO species which absorbs at 1740 cm-I might be involve~ in the formation of chemisorbed CO. When the flow of a i r was restored, the infrarec -I band centered at 2052 cm was immediately reduced in i n t e n s i t y . The infrared spectra of the species present on the surface during the oxidation of acetaldehyde at I00°C are shown in Figure 10.

Only one strong band centered at

1722 cm-I is observed in the infrared spectrum. There is no evidence of the surface ethoxy

species observed during the o x i d a t i o n of ethanol. Infrared bands due to

C-H stretching v i b r a t i o n s were not observed in the infrared spectrum.

6'7

-I~A

RU/SIO

2

5102 I

3200

28~

2540

2210 (CM -I)

1880

|5~

1220

FIGURE 10 I n s i t u infrared spectra obtained during the o x i d a t i o n of acetaldehyde on Pt/SiO 2, Ru/SiO 2 and SiO2 at I00°C. DISCUSSION The product d i s t r i b u t i o n obtained at low temperature suggests the following possible reaction scheme f o r the formation of acetaldehyde:

(i)

~02(g) + S ÷ O-S CH3CH2OH(g) + 2S ÷ CH3CH2-O-S + H-S CH3CH2-O-S

÷ CH3CHO(g) + H-S

2H-S + O-S

÷ H20 + 3S

(2) (3)

(4)

When the o x i d a t i o n of ethanol was carried out over the Ru/SiO 2 c a t a l y s t , acetaldehyde and H20 were the only reaction products formed. Carbon dioxide and acetic acid were formed only in trace amounts. Additional evidence for the above mechanism comes from the fact that the r a t i o of acetaldehyde to H20 was u n i t y and did not change over the 40-I00°C temperature range. Evidently the acetaldehyde formed as a r e s u l t of the oxidation of ethanol was not oxidized to other reaction products on Ru/SiO 2 in the 40-I00°C temperature range. The i n s i t u infrared studies reinforce t h i s conclusion. The i n f r a r e d spectrum presented in Figure 8 conclusively shows that a surface acetate is not formed during the oxidation of ethanol in e i t h e r Ru/SiO 2 or on pure s i l i c a .

In addition to t h i s observation, when acetaldehyde -I was exposed to a i r over Ru/SiO 2 the strong absorption at 1722 cm was absent and no reaction products were observed. On the other hand, the oxidation of acetaldehyde -I over Pt/SiO 2 lead to the formation of an intense infrared band centered at 1722 cm (Figure 10) in addition to formation of large amounts of acetic acid, and C02 [ 8 ] .

68 Ru/SiO2, therefore, appears to be an e x c e l l e n t choice of c a t a l y s t f o r the p a r t i a l oxidation of ethanol to acetaldehyde at low temperatures. When the o x i d a t i o n of ethanol was carried out over e i t h e r Pt/SiO 2, or Pt-Ru/Si02~ the rate of formation of acetaldehyde went through a maximum at 50°C on Pt/SiO 2 (70' on Pt-Ru/SiO 2

and then decreased to zero at about I00°C. A d d i t i o n a l l y , large

amounts of CO2 were observed in the reaction products at temperatures in excess of 50°C. Acetic acid and ethyl acetate were formed at higher temperatures. In order to account f o r t h i s d i s t r i b u t i o n of high temperature reaction products we suggest that the adsorbed ethoxy species can undergo f u r t h e r reaction with e i t h e r chemisorbc or l a t t i c e oxygen to form an adsorbed acetate species, as follows: CH

o'C% CH3CH2-O-S + O-S + 2S

S

S

+ 2H-S

C

o7 %o I S

S

The adsorbed acetate species can then add e i t h e r a proton, or react d i r e c t l y with chemisorbed oxygen to form CO2. S i m i l a r l y , ethyl acetate can form as the r e s u l t of a d i r e c t reaction between a surface ethoxy

and a surface acetate species.

This mechanism can lead to the d i r e c t o x i d a t i o n of ethanol to CO2 and H20 without the formation of acetaldehyde as an intermediate. The intermediates suggeste, here are e n t i r e l y consistent with observations made by Canning and Madix [12] and by Madix [13] regarding the adsorption of alcohols on well defined single crystal planes. We are, t h e r e f o r e , suggesting that CO2 can be formed by a p a r a l l e l reaction network e i t h e r through an acetaldehyde intermediate or by the d i r e c t oxidation of ethanol to CO2. In a separate study [8] we have reported on the oxidation of acetaldehyde over Pt/SiO 2 catalysts. In that study, very large amounts of acetic acid were observed. In fa c t , because i t was the primary reaction product, we feel that i f the high temperature reaction were to proceed p r i m a r i l y through an acetaldehyde intermediate, the amount of acetic acid formed would be considerably larger than that a c t u a l l y found in this study. The oxidation of ethanol over Pd/SiO2 and over a potassium promoted Rh/SiO2 catalyst showed much lower oxidation a c t i v i t y than on e i t h e r Pt/SiO 2, Ru/SiO2 or Pt-Ru/SiO 2. However, when the reaction was studied at higher temperatures, product d i s t r i b u t i o n s were q u i t e s i m i l a r to those on Pt/SiO 2.

69

The s i m i l a r i t y in the c a t a l y t i c behavior of Pt/SiO 2 and Pt-Ru/SiO 2 can be a t t r i b u t e d to the strong surface enrichment in Pt. Although the overall composition of the Pt-Ru b i m e t a l l i c cluster c a t a l y s t was 50% Pt, the surface composition was 75% Pt. We are not c e rt a i n whether the adsorbed acetate is located on the metal, on the support or on both. We think that the l a t t e r may be the most l i k e l y . We do know that the acetate is only formed to a s i g n i f i c a n t extent in the presence

of Pt.

However, i t may very well migrate to the support f o l l o w i n g i t s formation. Solymosi et al. [14] have recently studied the hydrogen assisted d i s s o c i a t i o n of CO2 on several s i l i c a and alumina supported noble metal catalysts. These workers established that the surface formate species were p r i m a r i l y adsorbed on the support. When alumina was used as a support, strong infrared bands assigned to surface formate species were observed. However, the i n t e n s i t y of these was s i g n i f i c a n t l y reduced when the noble metals were supported on s i l i c a , suggesting that perhaps a s i g n i f i cant f r a c t i o n of the surface acetate species in our study may reside on the Pt. The surface ethoxy

species are most c e r t a i n l y adsorbed on both the support and

the metal (Figure 8). We d i r e c t one f i n a l comment to the i n s i t u infrared spectra of the oxidation of acetaldehyde on Pt/SiO 2. Although the infrared band assigned to the surface acetate species at 1722 cm-I is well defined, there are no infrared bands which can be assigned to C-H containing species. This suggests that the adsorbed acetate must be positioned on the surface in a configuration in which the CH stretching modes are ina ctive , most possibly p a r a l l e l to the surface. Because of t h i s observation, there is no doubt t h a t the infrared bands centered at 2985, 2938 and 2906 cm-I are due s o l e l y to surface ethoxy

species with l i t t l e

or no c o n t r i b u t i o n from the methyl

group in the surface acetate. CONCLUSIONS The f o l l o w i n g conclusions emerge from this study: (I) Platinum was, by f a r , the most a c t i v e c a t a l y s t f o r the o x i d a t i o n of ethanol to CO2 and H20. (2) Ruthenium was very selective in the p a r t i a l oxidation of ethanol to acetaldehyde. (3) In s i t u i n f r a r e d studies suggest the presence of an acetate intermediate in addition to an adsorbed ethoxy species and chemisorbed CO. The ethoxy species are adsorbed both on Pt and on the s i l i c a support. (4) Two p a r a l l e l mechanisms appear to be involved. In the low temperature regime, ethanol is oxidized to acetaldehyde and H20. At higher temperatures the most important mechanism appears to be the d i r e c t oxidation of e~hanol to CO2 and H20 without the formation of acetaldehyde as an intermediate. This conclusion is based on the r e l a t i v e l y small amount of acetic acid that is formed. The high temperature o x i d a ti o n of acetaldehyde leads to very large amounts of acetic acid.

70 ACKNOWLEDGEMENTS Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of t h i s research. We also wish to acknowledge the Support of the National Science Foundation who provided funds f o r the purchase of the FTIR spectrophotometer used in t h i s research under Grant CHE-8216482o REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14

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