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Effect of the catalytic oxidation of ethylene, carbon monoxide and hydrogen on the chemisorption behavior of silver powder
SOURNAL OF COLLOID AND INTERFACE SCIENCE
24, 500-507 (1967)
Effect of the Catalytic Oxidation of Ethylene, Carbon Monoxide and Hydrogen on the Chemisorption Behavior of Silver Powder A. W . C Z A N D E R N A 1
Union Carbide Corporation, Chemicals and Plastics, Research and Development Department, South Charleston, West Virginia 25303 Received February 1, 1967; revised June 10, 1967 ABSTRACT
A study of the effect of the catalytic oxidation of mixtures of oxygen and hydrogen, ethylene, and carbon monoxide on a silver surface, reproducible to chemisorption behavior, has been carried out using a vacuum ultramicrobalance. Silver powder was subjected to OAOR ~ cycling in situ until reproducible chemisorption behavior was obtained. The powder was then used to catalyze the oxidation of hydrogen, ethylene, and carbon monoxide by allowing oxygen and the reducing gas to mix by diffusion. Measurements of the surface area and chemisorption behavior of the sample after each reaction indicate that surface disorder is increased by the reactions in the order H~ > C2H4 >> CO. The disordered silver surface could be re~ stored to a state of reproducible chemisorption behavior by extended OAOR cycling. I t is concluded from this work that during a vigorous catalytic reaction, new surface is being continually exposed and old surface buried in a churning fashion. This action tends to disorder the surface but is compensated by an increased rate of ordering of the high-energy surface formed and an increase in the surface temperature from the heat of reaction. The silver surface ultimately reaches a:steady state amount of disorder depending on the rate of reaction and the ambient temperature of the material. INTRODUCTION Numerous investigators have demons t r a t e d t h e d e p e n d e n c e of t h e c a t a l y t i c r e a c t i v i t y on t h e s t r u c t u r e a n d cleanness of a solid surface. T h u s , t h e p r e p a r a t i o n of a c l e a n well-defined solid c r y s t a l l i n e surface is o f t e n c o n s i d e r e d to be t h e essential first s t e p t o c o r r e l a t i n g surface s t r u c t u r e a n d c a t a lytic reactivity. However, unambiguous 1 Present address: Department of Physics and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, New York, 13676. OAOR cycling is repeated exposure of the sample to outgassing, oxygen adsorption, outgassing, and chemical reduction at a constant elevated temperature using the same ambient pressure in each adsorption and reduction step.
i n t e r p r e t a t i o n of studies c a r r i e d o u t on a surface of this t y p e c a n be a t t a i n e d o n l y for r e a c t i o n s in which t h e surface is n o t altered. U n f o r t u n a t e l y , t h e n a t u r e of a surface can be c h a n g e d b y t h e effect of kinetic factors such as c o n d e n s a t i o n , e v a p o ration, a n d c h e m i c a l reactions. I f t h e k i n e t i c f a c t o r s are negligible, t h e n u n d e r t h e p r o p e r c o n d i t i o n s of surface a t o m t r a n s p o r t a n d r e a r r a n g e m e n t , a final surface m a y a t t a i n the equilibrium structure that corresponds to a s t a t e of m i n i m u m free e n e r g y (1). F o r m a n y metals, this consists of t h e s m o o t h 111, 100, a n d 110 index p l a n e s a n d represents a n ideal surface. T h e p r o b l e m is t h a t in m a n y real c a t a l y t i c systems, kinetic factors are not negligible because high t e m p e r a tures are e m p l o y e d for c o m m e r c i a l reasons. 500
SURFACE CATALYTIC OXIDATION REACTIONS Thus, the difficulty of keeping a well-defined surface during the actual conditions of catalysis becomes obvious. Numerous methods can be used to detect changes in the stability of a surface (2). For example, a microbalance can be used to determine activation energies and heats of adsol~tion. However, little use has been made of the microbalance (3) since Rhodin carried out a number of basic studies (4). In the present study, a vacuum ultramicrobalance was used to measure mass changes of a low-energy silver surface before, during, and after use of the material as a catalyst for several simple oxidation reactions at temperatures where silver is a good catalyst. The gross stability of the silver was monitored by measurement of the BET surface area while subtle changes in the orientation of the surface atoms were inferred by measurement of the change in chemisorption behavior 3 of the silver surface. In previous work, it was shown that OAOR cycling eventually produces a cleaned silver surface which has a constant surface area, sample mass, and reproducible ehemisorption behavior (5). The cleaned surface was not poisoned by diffusion from the bulk and reaction products from the chemical reduction were not adsorbed on it (5, 6). The changes in the measured parameters of the surface area, sample mass, saturation oxygen uptake, amount of desorbable oxygen, and the rate of adsorption, desorption, and reduction result from the elimination of the most active adsorption sites by rearrangement of the silver surface during OAOP~ cycling (5).Arguments that the surface of the silver powder experiences a surface rearrangement to a low-energy surface, perhaps 111 and i00 index planes, during the OAOR cycling have been advanced (5). In addition, a detailed study of the adsorption, desorption, and thermodesorption of oxygen has been reported (7). Both reproducibility of the rate of chemisorption and of the saturation uptake at the same temperature and pressure are included in the term "ehemisorption behavior" and both must be the same if the sample surface reaches a stable configuration and is reprodueibly cleaned by the reduction treatment.
501
Thus, the ehemisorption properties of silver powder could be used, because of extensive prior studies, to detect subtle changes in the low-energy surface after use as a catalyst for several simple oxidation reactions. Silver is a good catalyst for the selective oxidation of ethylene to ethylene oxide at elevated temperatures (8). Recently, attempts have been made to correlate the catalytic reactivity of silver with its surface structure (9). These studies were carried out at temperatures below the region of industrial importance, viz., 250 ° to 400°C. Ethylene oxidation reactions on silver single crystals at 250°C. change the low index plane orientations to a polyerystalline surface (10). On the other hand, heat treatment of high-area powdered catalysts at progressively higher temperatues of 250 ° to 420°C. decreased the time required to reach the maximum selectivity of silver for the oxidation of ethylene to ethylene oxide. Since the higher temperatures simply sinter the surface, it was evident that sintering occurred during the early stages of the catalytic reaction (11). This indicates that a lower energy surface was produced during the reaction on powders. These seemingly conflicting reports can be explained by the studies reported below and by previous work (5). EXPERIMENTAL A vacuum ultramicrobalanee, which is particularly adapted for studying the sorption properties of low-area materials over broad ranges of temperature and pressure, was chosen for this study. The operation of the balance (7, 12) and the vacuum system (7), the preparation of the gases (7), the temperature control units (7), and the methods of study (5-7, 13) have been described. Silver powder was subiected to OAOR cycling in 20 tort of hydrogen or carbon monoxide at 390°C. until a surface reproducible to oxygen chemisorption was obtained (5). An OAOR cycle is shown schematically by Fig. i for a surface that exhibits reproducible ehemisorption behavior. Typieal changes in the parameters (1), (2), and (3) of Fig. 1 on extended OAOR cycling are shown in Fig. 2 from previous work (5, 7).
502
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Out-I Oxygen I Outgassing I Reduction l ~ gassing (adsorption) (desorption) (CO,Hz,CzH4)~ One OAOR cycle J" I
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FIG. 1. A typical OAOR cycle of silver powder using a vacuum ultramicrobalance. (1) Amount of mass gained by oxygen adsorption for 30 minutes; (2) amount of mass desorbed in 30 minutes;
(3) amount of mass lost on chemical reduction. In the early stages of cycling, the mass lost during (2) and (3) exceeded the mass gain during (1). When the ehemisorption behavior became reproducible, the loss during (2) and (3) was always equal to the gain during (1). The aliquots of silver powder, that were taken from the same preparation used in previous work (5, 6, 7), weighed 1.55 g. Catalytic reactions were run on the silver powder at 390°C. In general, oxygen was sealed in the balance chamber, and then the desired pressure of a reducing gas, carbon monoxide, ethylene, or hydrogen, was admitted to the gas handling section of the vacuum system. On opening the valve separating the two chambers, the reaction between oxygen and the reducing gas was initiated in I0 to 20 minutes, viz., when the reducing gas had sufficient time to diffuse through the oxygen to the silver powder. In most cases, the reactions were allowed to continue until the reducing gas in the balance and gas handling chambers was exhausted. The change in pressure was used to estimate termination of the reaction. The technique of gaseous diffusion in a closed system is similar to that used for precision equilibrium studies in the thermomolecular flow region (14). The data obtained in the latter study allowed accurate estimates of the diffusion time to be made. After each reaction was completed or
terminated, the reaction products were evacuated and the surface area of the powder was determined by nitrogen adsorption at 78°K. by the BET method and the gravimetrie technique described (13). Then, OAOR cycling was resumed at 390°C. with carbon monoxide used as the reducing gas to measure the extent of the change in the chemisorption behavior of the sample and, if any change had occurred, to determine if the silver surface could be restored to reproducible ehemisorption behavior. To expedite the experimental work, only the amount lost after outgassing for 30 minutes was measured in every cycle; the additional slow losses by prolonged outgassing and/or subsequent reduction are combined as a loss during reduction. The validity of this technique for determining reproducible ehemisorption behavior was established during prior work (5, 7, 14). 160 P = 10 Torr 140 ::L o~ 120
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FIG. 2. Typical changes in the amount of mass change during adsorption, desorption, and chemical reduction during repeated OAOR cycling.
SURFACE CATALYTIC OXIDATION REACTIONS RESULTS AND DISCUSSION
50 - -
Each sample was subjected to forty OAOR cycles in hydrogen and carbon monoxide at 390°C. until a constant amount of oxygen was gained during adsorption, lost during outgassing, and lost during reduction. These amounts are shown in Fig. 3 for the final 20 cycles of the typical preparatory OAOR cycling. The surface area of the samples became constant at 0.075
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FIG. 3. The equilibrium OAOR parameters (1), (2), and (3) for reproducible chemisorption behavior for silver powders prepared for the study of the effect of simple catalytic reactions. Solid circles, (1); triangles, (2); open circles, (3).
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m2/g. The Effect of Oxidation of Carbon Monoxide on the Silver Surface. The oxidation of carbon monoxide was carried out using carbon monoxide-oxygen mixture in which oxygen diffused to the silver surface through the carbon monoxide; complete oxidation was ensured by the use of excess oxygen. The change in mass by the sample dm'ing
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FIG. 4:. The mass change o f silver powder
during the e~talytie oxidation of carbon monoxide. Curve 1: CO (20 tort) -F 02 (30 torr) -* COs (20 torr) Jr O~ (20 torr); Curve 2: CO (40 torr) qO2 (10 torr) --* C02 (20 torr) q- CO (20 torr). this reaction is shown by Curve 1 of Fig. 4. The initial dip, which occurred after the customary time for diffusion of oxygen to the surface, is a transient buoyancy effect resulting from the heat of reaction. I t can be seen that the sample gained 87 % of the mass normal for adsorption in pure oxygen within 30 minutes and that the coverage of oxygen on the sample approached the value for pure oxygen as the pressure of CO dropped. F r o m the measured decrease in pressure in the system, it was evident that the oxidation of CO was quantitative. T h e small effect of this reaction on the chemisorption behavior of the silver powder is shown by Fig. 5a. The oxidation of carbon monoxide was then carried out using a carbon monoxideoxygen mixture in which the carbon monoxide diffused to the silver surface through oxygen; complete reduction was ensured by use of excess carbon monoxide. The change in mass by the sample during this reaction is shown by Curve 2 of Fig. 4. The customaW time of diffusion was not observed here because turbulent mixing of the two gases occurred during the admission of carbon monoxide to the sample from a relatively
504
CZANDERNA (a) 40L°
the sample was not changed from its original value of 0.075 m ? / g . by either of the above reactions.
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the catalytic oxidation of carbon monoxide by silver powder. (a) After oxidation in an oxygen rich mixture. (b) After oxidation in a carbon monoxide rich mixture. Solid circles, (•); Triangles, (9); Open Circles, (8). The error in the mass measurement is about one tenth the size of the circles and triangles.
The Effect of Oxidation of Ethylene on the Silver Surface. T h e oxidation of ethylene was carried out using an ethylene-oxygen mixture in which ethylene diffused to the silver surface through the oxygen. T h e initial change in mass b y the sample during this reaction is shown in Fig. 6. T h e sample
mass remained essentially unaltered for nearly an hour after allowing for the customary time for diffusion of ethylene to the surface. The sample then started to gain mass at an increasing rate. After 45 hours, nearly 500 ~g. was gained and the sample mass was increasing linearly. This is shown by Fig. 7; the temperature of the silver during subsequent experimentation is also plotted in this figure. The reaction was slowed and then terminated by removal of the ethylene-oxygen mixture in pressure decrements at 390°C. In the oxidation of carbon monoxide, the reaction to form carbon dioxide was quantitative as measured by the total pressure drop of i0 torr; in the present reaction, a decrease in pressure of only 2 torr was noted before the reaction was stopped. A decrease in pressure of i0 torr would have resulted if all the ethylene had formed ethylene oxide; a decrease in pressure could result if the large mass gain by the sample was a carbonaceous 50
high pressure. I t can be seen t h a t more time was required for carbon monoxide to reduce the major fraction of the oxygen from the surface t h a n for oxygen to populate the surface in the presence of carbon monoxide. I t can also be seen from the rapid approach of the sample mass to the value characteristic of a reduced silver surface t h a t the complete depletion of oxygen took less time t h a n the complete coverage b y oxygen in the presence of carbon monoxide. Again, the effect on the chemisorption behavior was slight, as shown in Fig. 5b. Only three to five OAOR cycles were needed to restore the silver powder to reproducible chemisorption behavior. T h e surface area of
25 k,:
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FIG. 6. The mass change of silver powder during the early stages of the catalytic oxidation of ethylene in an oxygen rich mixture, i.e., C~H~ (I0 torr) d- O~ (40 torr) -+ carbonaceous products -4- gaseous products Jr Os.
SURFACE CATALYTIC OXIDATION REACTIONS
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Fro. 7. The mass change of silver powder during the catalytic oxidation of ethylene and in subsequent thermochemieal treatments. complex. Complete oxidation of ethylene to carbon dioxide and water would not change the pressure. To account for the observed decrease in pressure, reactions in which both ethylene oxide and the carbonaceous complex were formed had to occur. No further analysis of the reaction products was attempted. Prolonged outgassing of the sample at 390°C. removed only 1% of the mass gain (Fig. 7). When the sample was cooled to room temperature and exposed to oxygen, a mass gain was noted. The sample was heated progressively to higher temperatures in oxygen until a mass loss was experienced by the sample when the temperature exceeded 175°C. However, only 40 % of the mass gained in ethylene was removed even after exposure to prolonged oxidizing conditions at 390°C. When the sample was subjeeted to OAOR eyeling, the sample mass did not decrease significantly over twenty cycles. The change in the ehemisorption behavior of the sample was influenced dramatically after the catalytic reaction of ethylene as can be seen in Fig. 8. About fifteen cycles were required to restore the sample to a state of normal ehemisorption behavior, compared with three to five for the surface
505
disturbed by carbon monoxide oxidation. I t was surprising that normal chemisorption behavior could be regained after this reaction, because much of the large mass gain in ethylene was still retained by the sample. The nature of the mass retained by the sample is not known. If it consisted entirely of a carbonaceous complex on the surface, it could be burned off by oxygen (15). The mass could not be removed by exposure to oxygen at 390°C. and its presence did not produce any detectable influence on the ehemisorption behavior of well-cycled silver. Thus, it is possible the material was trapped beneath the surface of the silver during the reaction. This result could be obtained if the carbonaceous material was churned under the silver surface during the reaction. The conditions of OAOR cycling then would be sufficient to regenerate a reproducible surface over the trapped material but 40
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506
CZANDERNA
would not be severe enough to churn the \ surface to permit its elimination. It is not impossible that an unreactive material is formed on the surface that could not be detected as an increase in the surface area or a change in the chemisorption behavior. ~20 ~ • ~ The surface area was increased by approxio o ~ ~ A A A a ~ A a ~ ~ n mately 10 % by the ethylene reaction. After aAA A A ,5. AA A A A ^ A OAOR cycling, it decreased to 0.076 m.Vg., which is within experimental error of the o o o o o coo ~ Q - ~ o ~ - original value. Further microanalytical work o./ is necessary to elucidate the nature of the o o o / o mass gained by the silver during the earnc i o i . I l l I G¢ 5 I0 15 20 25 30 35 lyric oxidation of ethylene under the eondiOAOR CYCLE NUMBER tions of the above experiment. FxG. i0. The change in O A O R parameters The Effect of the Oxidation of Hydrogen on after the catalytic oxidation of hydrogen by silver a Silver Surface. The oxidation of hydrogen powder in an oxygen rich mixture. Solid eire]es, was carried out using a hydrogen-oxygen (•); triangles, (2); open circles, (3). mixture in which hydrogen diffused to the silver surface through the oxygen; complete oxidation was ensured by the use of excess hydrogen and oxygen was completed, as oxygen. The change in mass by the sample noted from the pressure decrease in the during this reaction is shown by Fig. 9. system. Evacuation of the mixture followed After allowing for the customary time for by outgassing did not remove any of the hydrogen to diffuse to the silver surface, mass gained during the reaction. This type which, of course, was shorter than for carbon of behavior was observed previously for monoxide and ethylene, the sample gained high-energy silver surfaces on which oxygen approximately 50 ~g. in mass. The gain in is ehemisorbed strongly (15). The surface mass by the sample terminated at approxi- area of the silver was increased during the mately the same time the reaction between reaction by about 10%. When the sample was subjected to OAOR cycling, as shown by Fig. 10, the chemisorption behavior of 60 the sample exhibited marked changes. Not only was it irreprodueible when hydrogen 50 was used in place of carbon monoxide in the OAOR cycle (Fig. 10), but also it depended on the outgassing time. The changes in 40 ehemisorption behavior observed apparently are the result of trapping or sealing oxygen 2o 3O .m_ and/or water in pores beneath the surface during the catalytic reaction. The trapped 10 c~ gas was apparently removed gradually by 20 g H2 bulk diffusion or in pulses if a violent reducI tion produced surface rearrangement that I0 exposed the trapped gases. In any event, it is clear from Fig. 10 that the ehemisorption behavior of the silver powder was changed dramatically after use of a silver surface to -IC 1 I catalyze the oxidation of hydrogen. I00 200 500 t (rnin) The extended OAOR cycling after the hydrogen-oxygen reaction ultimately generFIG. 9. The mass change of silver powder ated a surface reproducible to oxygen ehemiduring the catalytic oxidation of hydrogen in an oxygen rich mixture, i.e., H2 (20 tort) + Oe (30 sorption even though the amount of adsorption at saturation was decreased. The torr) --~ H20 (20 torr) + O2 (20 tort). E
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SURFACE CATALYTIC OXIDATION REACTIONS surface area of the sample was returned to its original value by cycling. The rate of oxygen adsoi~ption was studied then at a series of temperatures ranging from 25 ° to 250°C. to establish the activation energies for oxygen adsorption on this reproducible surface. The values found exhibited the same dependence on surface coverage as reported previously for silver powders (7). Since these data are the same, they will not be reported again; it is suffÉcient to note that the chemisorption behavior of this surface exhibited adsorption, desorption, thermodesorption, and activation energy properties comparable with those found and repol%ed earlier for silver powder with reproducible chemisorption behavior (7). Disordering of the Surface during Catalytic Reactions. It is evident from the results of
the simple oxidation reactions that kinetic factors are not negligible at temperatures where silver is a good catalyst for the three oxidation reactions studied. Thus, the lowenergy surface generated by careful OAOR cycling is not retained during the catalytic reactions investigated. The oxidation of carbon monoxide had a relatively small effect, but the ethylene and hydrogen reactions had strong disturbing effects on the surface. The interactions between the surface atoms and the gases, the heat effects, and other kinetic factors resulted in a movement of the surface atoms, a disordering of the surface, and the production of a higher energy surface. This new surface did not exhibit reproducible chemisorption behavior, but with sufficient OAOR cycling the disturbed surface could be restored to chemisorptive reproducibility. Finally, it is of interest to resolve the apparently conflicting results by Wilson et al. (10) and Echigoya et al. (11). In the first study the reaction was carried out on a lowenergy single crystal surface. The surface generated was one of higher energy as evidenced by the formation of polycrystalline silver. In the second study, heat treatment of a powdered silver catalyst resulted in a decrease in the surface energy. The surface generated was close to the type of surface present during the catalytic reaction under steady-state conditions. The changes observed in the chemisorption behavior of
507
silver powder during the early stages of OAOR cycling (5) result from a lowering of the surface energy, similar to the effects found by Echigoya, Amberg, and Osberg. However, after a low-energy surface is generated by OAOR cycling, the effect of catalytic reactions is to increase the surface energy; this is similar to the results obtained by Wilson and co-workers (10). ACKNOWLEDGMENTS The author is pleased to acknowledge the fruitful discussions with Dr. H. C. Chitwood concerning this problem. The assistance of Mr. P. A. Given, who carried out the microgravimetric measurements, is also appreciated. Finally, the author is grateful to Dr. Stephen Brunauer for his encouragement and many helpful comments about this work. REFERENCES 1. For example, see BRENNER, S., Surface Sei. 2, 496 (1964) and references therein. 2. HAYWARD, D. O., AND TRAPNELL, B. M. W., "Chemisorption," 2nd ed., Butterworths, London, 1964. 3. CZANDERNA,A. W., "Ultramicrobalance Review" in S. P. Wolsky and E. J. Zdanuk, eds., "Ultramicroweighing in Controlled Environments." Wiley, New York, to be published. 4. RHODIN, W. N., Advan. Catalysis 5, 39 (1953). 5. CZANDERNA, A. W., J. Phys. Chem. 70, 2120 (1966). 6. CZANDERNA, A. W., J. Colloid and Interface Sei. 22,482 (1966). 7. CZANDERNA, A. W., J. Phys. Chem. 68, 2765 (1964). 8. TWIGG, G. H., Trans. Faraday Soc. 42, 284 (1946). 9. SANDERS, J. V., J. Australian Inst. Metals 9, 63 (1964); Chem. Abstr. 61, 8935g. 10. WILSON, J. N., VOGE, H. H., STEVENSON, D. P., SMITH, A. E., ANn ATKINS, L. T., J. Phys. Chem. 63, 463 (1959). 11. ECHIGOYA, E., AMBERG, C. ~-L, AND OSBERG, G. L., Can. J. Chem. 37, 2101 (1959). 12. CZANDERNA, A. W., I n P. M. Waters, ed., "Vacuum Microbalance Techniques," Vol. 4, p. 57. Plenum Press, New York, 1965. 13. CZANDERNA, A. W., AND •ONIG, J. M., g. Phys. Chem. 63, 620 (1959). 14. CZANDERNA, A. W., I n P. M. Waters, ed., "Vacuum Microbalance Techniques," Vol. 4, p. 69. Plenum Press, New York, 1965. 15. CZANDERNA,A. W., Unpublished results.
24, 500-507 (1967)
Effect of the Catalytic Oxidation of Ethylene, Carbon Monoxide and Hydrogen on the Chemisorption Behavior of Silver Powder A. W . C Z A N D E R N A 1
Union Carbide Corporation, Chemicals and Plastics, Research and Development Department, South Charleston, West Virginia 25303 Received February 1, 1967; revised June 10, 1967 ABSTRACT
A study of the effect of the catalytic oxidation of mixtures of oxygen and hydrogen, ethylene, and carbon monoxide on a silver surface, reproducible to chemisorption behavior, has been carried out using a vacuum ultramicrobalance. Silver powder was subjected to OAOR ~ cycling in situ until reproducible chemisorption behavior was obtained. The powder was then used to catalyze the oxidation of hydrogen, ethylene, and carbon monoxide by allowing oxygen and the reducing gas to mix by diffusion. Measurements of the surface area and chemisorption behavior of the sample after each reaction indicate that surface disorder is increased by the reactions in the order H~ > C2H4 >> CO. The disordered silver surface could be re~ stored to a state of reproducible chemisorption behavior by extended OAOR cycling. I t is concluded from this work that during a vigorous catalytic reaction, new surface is being continually exposed and old surface buried in a churning fashion. This action tends to disorder the surface but is compensated by an increased rate of ordering of the high-energy surface formed and an increase in the surface temperature from the heat of reaction. The silver surface ultimately reaches a:steady state amount of disorder depending on the rate of reaction and the ambient temperature of the material. INTRODUCTION Numerous investigators have demons t r a t e d t h e d e p e n d e n c e of t h e c a t a l y t i c r e a c t i v i t y on t h e s t r u c t u r e a n d cleanness of a solid surface. T h u s , t h e p r e p a r a t i o n of a c l e a n well-defined solid c r y s t a l l i n e surface is o f t e n c o n s i d e r e d to be t h e essential first s t e p t o c o r r e l a t i n g surface s t r u c t u r e a n d c a t a lytic reactivity. However, unambiguous 1 Present address: Department of Physics and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, New York, 13676. OAOR cycling is repeated exposure of the sample to outgassing, oxygen adsorption, outgassing, and chemical reduction at a constant elevated temperature using the same ambient pressure in each adsorption and reduction step.
i n t e r p r e t a t i o n of studies c a r r i e d o u t on a surface of this t y p e c a n be a t t a i n e d o n l y for r e a c t i o n s in which t h e surface is n o t altered. U n f o r t u n a t e l y , t h e n a t u r e of a surface can be c h a n g e d b y t h e effect of kinetic factors such as c o n d e n s a t i o n , e v a p o ration, a n d c h e m i c a l reactions. I f t h e k i n e t i c f a c t o r s are negligible, t h e n u n d e r t h e p r o p e r c o n d i t i o n s of surface a t o m t r a n s p o r t a n d r e a r r a n g e m e n t , a final surface m a y a t t a i n the equilibrium structure that corresponds to a s t a t e of m i n i m u m free e n e r g y (1). F o r m a n y metals, this consists of t h e s m o o t h 111, 100, a n d 110 index p l a n e s a n d represents a n ideal surface. T h e p r o b l e m is t h a t in m a n y real c a t a l y t i c systems, kinetic factors are not negligible because high t e m p e r a tures are e m p l o y e d for c o m m e r c i a l reasons. 500
SURFACE CATALYTIC OXIDATION REACTIONS Thus, the difficulty of keeping a well-defined surface during the actual conditions of catalysis becomes obvious. Numerous methods can be used to detect changes in the stability of a surface (2). For example, a microbalance can be used to determine activation energies and heats of adsol~tion. However, little use has been made of the microbalance (3) since Rhodin carried out a number of basic studies (4). In the present study, a vacuum ultramicrobalance was used to measure mass changes of a low-energy silver surface before, during, and after use of the material as a catalyst for several simple oxidation reactions at temperatures where silver is a good catalyst. The gross stability of the silver was monitored by measurement of the BET surface area while subtle changes in the orientation of the surface atoms were inferred by measurement of the change in chemisorption behavior 3 of the silver surface. In previous work, it was shown that OAOR cycling eventually produces a cleaned silver surface which has a constant surface area, sample mass, and reproducible ehemisorption behavior (5). The cleaned surface was not poisoned by diffusion from the bulk and reaction products from the chemical reduction were not adsorbed on it (5, 6). The changes in the measured parameters of the surface area, sample mass, saturation oxygen uptake, amount of desorbable oxygen, and the rate of adsorption, desorption, and reduction result from the elimination of the most active adsorption sites by rearrangement of the silver surface during OAOP~ cycling (5).Arguments that the surface of the silver powder experiences a surface rearrangement to a low-energy surface, perhaps 111 and i00 index planes, during the OAOR cycling have been advanced (5). In addition, a detailed study of the adsorption, desorption, and thermodesorption of oxygen has been reported (7). Both reproducibility of the rate of chemisorption and of the saturation uptake at the same temperature and pressure are included in the term "ehemisorption behavior" and both must be the same if the sample surface reaches a stable configuration and is reprodueibly cleaned by the reduction treatment.
501
Thus, the ehemisorption properties of silver powder could be used, because of extensive prior studies, to detect subtle changes in the low-energy surface after use as a catalyst for several simple oxidation reactions. Silver is a good catalyst for the selective oxidation of ethylene to ethylene oxide at elevated temperatures (8). Recently, attempts have been made to correlate the catalytic reactivity of silver with its surface structure (9). These studies were carried out at temperatures below the region of industrial importance, viz., 250 ° to 400°C. Ethylene oxidation reactions on silver single crystals at 250°C. change the low index plane orientations to a polyerystalline surface (10). On the other hand, heat treatment of high-area powdered catalysts at progressively higher temperatues of 250 ° to 420°C. decreased the time required to reach the maximum selectivity of silver for the oxidation of ethylene to ethylene oxide. Since the higher temperatures simply sinter the surface, it was evident that sintering occurred during the early stages of the catalytic reaction (11). This indicates that a lower energy surface was produced during the reaction on powders. These seemingly conflicting reports can be explained by the studies reported below and by previous work (5). EXPERIMENTAL A vacuum ultramicrobalanee, which is particularly adapted for studying the sorption properties of low-area materials over broad ranges of temperature and pressure, was chosen for this study. The operation of the balance (7, 12) and the vacuum system (7), the preparation of the gases (7), the temperature control units (7), and the methods of study (5-7, 13) have been described. Silver powder was subiected to OAOR cycling in 20 tort of hydrogen or carbon monoxide at 390°C. until a surface reproducible to oxygen chemisorption was obtained (5). An OAOR cycle is shown schematically by Fig. i for a surface that exhibits reproducible ehemisorption behavior. Typieal changes in the parameters (1), (2), and (3) of Fig. 1 on extended OAOR cycling are shown in Fig. 2 from previous work (5, 7).
502
CZANDERNA
Out-I Oxygen I Outgassing I Reduction l ~ gassing (adsorption) (desorption) (CO,Hz,CzH4)~ One OAOR cycle J" I
g
(I)
k
(3)
Time (arbitrary units)
FIG. 1. A typical OAOR cycle of silver powder using a vacuum ultramicrobalance. (1) Amount of mass gained by oxygen adsorption for 30 minutes; (2) amount of mass desorbed in 30 minutes;
(3) amount of mass lost on chemical reduction. In the early stages of cycling, the mass lost during (2) and (3) exceeded the mass gain during (1). When the ehemisorption behavior became reproducible, the loss during (2) and (3) was always equal to the gain during (1). The aliquots of silver powder, that were taken from the same preparation used in previous work (5, 6, 7), weighed 1.55 g. Catalytic reactions were run on the silver powder at 390°C. In general, oxygen was sealed in the balance chamber, and then the desired pressure of a reducing gas, carbon monoxide, ethylene, or hydrogen, was admitted to the gas handling section of the vacuum system. On opening the valve separating the two chambers, the reaction between oxygen and the reducing gas was initiated in I0 to 20 minutes, viz., when the reducing gas had sufficient time to diffuse through the oxygen to the silver powder. In most cases, the reactions were allowed to continue until the reducing gas in the balance and gas handling chambers was exhausted. The change in pressure was used to estimate termination of the reaction. The technique of gaseous diffusion in a closed system is similar to that used for precision equilibrium studies in the thermomolecular flow region (14). The data obtained in the latter study allowed accurate estimates of the diffusion time to be made. After each reaction was completed or
terminated, the reaction products were evacuated and the surface area of the powder was determined by nitrogen adsorption at 78°K. by the BET method and the gravimetrie technique described (13). Then, OAOR cycling was resumed at 390°C. with carbon monoxide used as the reducing gas to measure the extent of the change in the chemisorption behavior of the sample and, if any change had occurred, to determine if the silver surface could be restored to reproducible ehemisorption behavior. To expedite the experimental work, only the amount lost after outgassing for 30 minutes was measured in every cycle; the additional slow losses by prolonged outgassing and/or subsequent reduction are combined as a loss during reduction. The validity of this technique for determining reproducible ehemisorption behavior was established during prior work (5, 7, 14). 160 P = 10 Torr 140 ::L o~ 120
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ac 80
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20
FIG. 2. Typical changes in the amount of mass change during adsorption, desorption, and chemical reduction during repeated OAOR cycling.
SURFACE CATALYTIC OXIDATION REACTIONS RESULTS AND DISCUSSION
50 - -
Each sample was subjected to forty OAOR cycles in hydrogen and carbon monoxide at 390°C. until a constant amount of oxygen was gained during adsorption, lost during outgassing, and lost during reduction. These amounts are shown in Fig. 3 for the final 20 cycles of the typical preparatory OAOR cycling. The surface area of the samples became constant at 0.075
40
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FIG. 3. The equilibrium OAOR parameters (1), (2), and (3) for reproducible chemisorption behavior for silver powders prepared for the study of the effect of simple catalytic reactions. Solid circles, (1); triangles, (2); open circles, (3).
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m2/g. The Effect of Oxidation of Carbon Monoxide on the Silver Surface. The oxidation of carbon monoxide was carried out using carbon monoxide-oxygen mixture in which oxygen diffused to the silver surface through the carbon monoxide; complete oxidation was ensured by the use of excess oxygen. The change in mass by the sample dm'ing
,
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FIG. 4:. The mass change o f silver powder
during the e~talytie oxidation of carbon monoxide. Curve 1: CO (20 tort) -F 02 (30 torr) -* COs (20 torr) Jr O~ (20 torr); Curve 2: CO (40 torr) qO2 (10 torr) --* C02 (20 torr) q- CO (20 torr). this reaction is shown by Curve 1 of Fig. 4. The initial dip, which occurred after the customary time for diffusion of oxygen to the surface, is a transient buoyancy effect resulting from the heat of reaction. I t can be seen that the sample gained 87 % of the mass normal for adsorption in pure oxygen within 30 minutes and that the coverage of oxygen on the sample approached the value for pure oxygen as the pressure of CO dropped. F r o m the measured decrease in pressure in the system, it was evident that the oxidation of CO was quantitative. T h e small effect of this reaction on the chemisorption behavior of the silver powder is shown by Fig. 5a. The oxidation of carbon monoxide was then carried out using a carbon monoxideoxygen mixture in which the carbon monoxide diffused to the silver surface through oxygen; complete reduction was ensured by use of excess carbon monoxide. The change in mass by the sample during this reaction is shown by Curve 2 of Fig. 4. The customaW time of diffusion was not observed here because turbulent mixing of the two gases occurred during the admission of carbon monoxide to the sample from a relatively
504
CZANDERNA (a) 40L°
the sample was not changed from its original value of 0.075 m ? / g . by either of the above reactions.
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5. The change in OAOI~ p a r a m e t e r s a f t e r
the catalytic oxidation of carbon monoxide by silver powder. (a) After oxidation in an oxygen rich mixture. (b) After oxidation in a carbon monoxide rich mixture. Solid circles, (•); Triangles, (9); Open Circles, (8). The error in the mass measurement is about one tenth the size of the circles and triangles.
The Effect of Oxidation of Ethylene on the Silver Surface. T h e oxidation of ethylene was carried out using an ethylene-oxygen mixture in which ethylene diffused to the silver surface through the oxygen. T h e initial change in mass b y the sample during this reaction is shown in Fig. 6. T h e sample
mass remained essentially unaltered for nearly an hour after allowing for the customary time for diffusion of ethylene to the surface. The sample then started to gain mass at an increasing rate. After 45 hours, nearly 500 ~g. was gained and the sample mass was increasing linearly. This is shown by Fig. 7; the temperature of the silver during subsequent experimentation is also plotted in this figure. The reaction was slowed and then terminated by removal of the ethylene-oxygen mixture in pressure decrements at 390°C. In the oxidation of carbon monoxide, the reaction to form carbon dioxide was quantitative as measured by the total pressure drop of i0 torr; in the present reaction, a decrease in pressure of only 2 torr was noted before the reaction was stopped. A decrease in pressure of i0 torr would have resulted if all the ethylene had formed ethylene oxide; a decrease in pressure could result if the large mass gain by the sample was a carbonaceous 50
high pressure. I t can be seen t h a t more time was required for carbon monoxide to reduce the major fraction of the oxygen from the surface t h a n for oxygen to populate the surface in the presence of carbon monoxide. I t can also be seen from the rapid approach of the sample mass to the value characteristic of a reduced silver surface t h a t the complete depletion of oxygen took less time t h a n the complete coverage b y oxygen in the presence of carbon monoxide. Again, the effect on the chemisorption behavior was slight, as shown in Fig. 5b. Only three to five OAOR cycles were needed to restore the silver powder to reproducible chemisorption behavior. T h e surface area of
25 k,:
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o-~ >z
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uxygen coverea silver surface i
O
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200 t (rnin)
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FIG. 6. The mass change of silver powder during the early stages of the catalytic oxidation of ethylene in an oxygen rich mixture, i.e., C~H~ (I0 torr) d- O~ (40 torr) -+ carbonaceous products -4- gaseous products Jr Os.
SURFACE CATALYTIC OXIDATION REACTIONS
26 6 Toi~lP=47~ 0-4 Torr 400
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Fro. 7. The mass change of silver powder during the catalytic oxidation of ethylene and in subsequent thermochemieal treatments. complex. Complete oxidation of ethylene to carbon dioxide and water would not change the pressure. To account for the observed decrease in pressure, reactions in which both ethylene oxide and the carbonaceous complex were formed had to occur. No further analysis of the reaction products was attempted. Prolonged outgassing of the sample at 390°C. removed only 1% of the mass gain (Fig. 7). When the sample was cooled to room temperature and exposed to oxygen, a mass gain was noted. The sample was heated progressively to higher temperatures in oxygen until a mass loss was experienced by the sample when the temperature exceeded 175°C. However, only 40 % of the mass gained in ethylene was removed even after exposure to prolonged oxidizing conditions at 390°C. When the sample was subjeeted to OAOR eyeling, the sample mass did not decrease significantly over twenty cycles. The change in the ehemisorption behavior of the sample was influenced dramatically after the catalytic reaction of ethylene as can be seen in Fig. 8. About fifteen cycles were required to restore the sample to a state of normal ehemisorption behavior, compared with three to five for the surface
505
disturbed by carbon monoxide oxidation. I t was surprising that normal chemisorption behavior could be regained after this reaction, because much of the large mass gain in ethylene was still retained by the sample. The nature of the mass retained by the sample is not known. If it consisted entirely of a carbonaceous complex on the surface, it could be burned off by oxygen (15). The mass could not be removed by exposure to oxygen at 390°C. and its presence did not produce any detectable influence on the ehemisorption behavior of well-cycled silver. Thus, it is possible the material was trapped beneath the surface of the silver during the reaction. This result could be obtained if the carbonaceous material was churned under the silver surface during the reaction. The conditions of OAOR cycling then would be sufficient to regenerate a reproducible surface over the trapped material but 40
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change in OAOR parameters after oxidation of ethylene by silver oxygen rich mixture. Solid circles, (2); open circles, (3).
506
CZANDERNA
would not be severe enough to churn the \ surface to permit its elimination. It is not impossible that an unreactive material is formed on the surface that could not be detected as an increase in the surface area or a change in the chemisorption behavior. ~20 ~ • ~ The surface area was increased by approxio o ~ ~ A A A a ~ A a ~ ~ n mately 10 % by the ethylene reaction. After aAA A A ,5. AA A A A ^ A OAOR cycling, it decreased to 0.076 m.Vg., which is within experimental error of the o o o o o coo ~ Q - ~ o ~ - original value. Further microanalytical work o./ is necessary to elucidate the nature of the o o o / o mass gained by the silver during the earnc i o i . I l l I G¢ 5 I0 15 20 25 30 35 lyric oxidation of ethylene under the eondiOAOR CYCLE NUMBER tions of the above experiment. FxG. i0. The change in O A O R parameters The Effect of the Oxidation of Hydrogen on after the catalytic oxidation of hydrogen by silver a Silver Surface. The oxidation of hydrogen powder in an oxygen rich mixture. Solid eire]es, was carried out using a hydrogen-oxygen (•); triangles, (2); open circles, (3). mixture in which hydrogen diffused to the silver surface through the oxygen; complete oxidation was ensured by the use of excess hydrogen and oxygen was completed, as oxygen. The change in mass by the sample noted from the pressure decrease in the during this reaction is shown by Fig. 9. system. Evacuation of the mixture followed After allowing for the customary time for by outgassing did not remove any of the hydrogen to diffuse to the silver surface, mass gained during the reaction. This type which, of course, was shorter than for carbon of behavior was observed previously for monoxide and ethylene, the sample gained high-energy silver surfaces on which oxygen approximately 50 ~g. in mass. The gain in is ehemisorbed strongly (15). The surface mass by the sample terminated at approxi- area of the silver was increased during the mately the same time the reaction between reaction by about 10%. When the sample was subjected to OAOR cycling, as shown by Fig. 10, the chemisorption behavior of 60 the sample exhibited marked changes. Not only was it irreprodueible when hydrogen 50 was used in place of carbon monoxide in the OAOR cycle (Fig. 10), but also it depended on the outgassing time. The changes in 40 ehemisorption behavior observed apparently are the result of trapping or sealing oxygen 2o 3O .m_ and/or water in pores beneath the surface during the catalytic reaction. The trapped 10 c~ gas was apparently removed gradually by 20 g H2 bulk diffusion or in pulses if a violent reducI tion produced surface rearrangement that I0 exposed the trapped gases. In any event, it is clear from Fig. 10 that the ehemisorption behavior of the silver powder was changed dramatically after use of a silver surface to -IC 1 I catalyze the oxidation of hydrogen. I00 200 500 t (rnin) The extended OAOR cycling after the hydrogen-oxygen reaction ultimately generFIG. 9. The mass change of silver powder ated a surface reproducible to oxygen ehemiduring the catalytic oxidation of hydrogen in an oxygen rich mixture, i.e., H2 (20 tort) + Oe (30 sorption even though the amount of adsorption at saturation was decreased. The torr) --~ H20 (20 torr) + O2 (20 tort). E
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,
SURFACE CATALYTIC OXIDATION REACTIONS surface area of the sample was returned to its original value by cycling. The rate of oxygen adsoi~ption was studied then at a series of temperatures ranging from 25 ° to 250°C. to establish the activation energies for oxygen adsorption on this reproducible surface. The values found exhibited the same dependence on surface coverage as reported previously for silver powders (7). Since these data are the same, they will not be reported again; it is suffÉcient to note that the chemisorption behavior of this surface exhibited adsorption, desorption, thermodesorption, and activation energy properties comparable with those found and repol%ed earlier for silver powder with reproducible chemisorption behavior (7). Disordering of the Surface during Catalytic Reactions. It is evident from the results of
the simple oxidation reactions that kinetic factors are not negligible at temperatures where silver is a good catalyst for the three oxidation reactions studied. Thus, the lowenergy surface generated by careful OAOR cycling is not retained during the catalytic reactions investigated. The oxidation of carbon monoxide had a relatively small effect, but the ethylene and hydrogen reactions had strong disturbing effects on the surface. The interactions between the surface atoms and the gases, the heat effects, and other kinetic factors resulted in a movement of the surface atoms, a disordering of the surface, and the production of a higher energy surface. This new surface did not exhibit reproducible chemisorption behavior, but with sufficient OAOR cycling the disturbed surface could be restored to chemisorptive reproducibility. Finally, it is of interest to resolve the apparently conflicting results by Wilson et al. (10) and Echigoya et al. (11). In the first study the reaction was carried out on a lowenergy single crystal surface. The surface generated was one of higher energy as evidenced by the formation of polycrystalline silver. In the second study, heat treatment of a powdered silver catalyst resulted in a decrease in the surface energy. The surface generated was close to the type of surface present during the catalytic reaction under steady-state conditions. The changes observed in the chemisorption behavior of
507
silver powder during the early stages of OAOR cycling (5) result from a lowering of the surface energy, similar to the effects found by Echigoya, Amberg, and Osberg. However, after a low-energy surface is generated by OAOR cycling, the effect of catalytic reactions is to increase the surface energy; this is similar to the results obtained by Wilson and co-workers (10). ACKNOWLEDGMENTS The author is pleased to acknowledge the fruitful discussions with Dr. H. C. Chitwood concerning this problem. The assistance of Mr. P. A. Given, who carried out the microgravimetric measurements, is also appreciated. Finally, the author is grateful to Dr. Stephen Brunauer for his encouragement and many helpful comments about this work. REFERENCES 1. For example, see BRENNER, S., Surface Sei. 2, 496 (1964) and references therein. 2. HAYWARD, D. O., AND TRAPNELL, B. M. W., "Chemisorption," 2nd ed., Butterworths, London, 1964. 3. CZANDERNA,A. W., "Ultramicrobalance Review" in S. P. Wolsky and E. J. Zdanuk, eds., "Ultramicroweighing in Controlled Environments." Wiley, New York, to be published. 4. RHODIN, W. N., Advan. Catalysis 5, 39 (1953). 5. CZANDERNA, A. W., J. Phys. Chem. 70, 2120 (1966). 6. CZANDERNA, A. W., J. Colloid and Interface Sei. 22,482 (1966). 7. CZANDERNA, A. W., J. Phys. Chem. 68, 2765 (1964). 8. TWIGG, G. H., Trans. Faraday Soc. 42, 284 (1946). 9. SANDERS, J. V., J. Australian Inst. Metals 9, 63 (1964); Chem. Abstr. 61, 8935g. 10. WILSON, J. N., VOGE, H. H., STEVENSON, D. P., SMITH, A. E., ANn ATKINS, L. T., J. Phys. Chem. 63, 463 (1959). 11. ECHIGOYA, E., AMBERG, C. ~-L, AND OSBERG, G. L., Can. J. Chem. 37, 2101 (1959). 12. CZANDERNA, A. W., I n P. M. Waters, ed., "Vacuum Microbalance Techniques," Vol. 4, p. 57. Plenum Press, New York, 1965. 13. CZANDERNA, A. W., AND •ONIG, J. M., g. Phys. Chem. 63, 620 (1959). 14. CZANDERNA, A. W., I n P. M. Waters, ed., "Vacuum Microbalance Techniques," Vol. 4, p. 69. Plenum Press, New York, 1965. 15. CZANDERNA,A. W., Unpublished results.