J. Phys. Chem. 1981, 85, 434-439
434
an attempt to answer the question as to which value of S (298 K) is correct, we have calculated the statistical thermodynamic functions of 3-methyl-1-butene using both VCH3= 4.2 kcal/moP’ and VCHsas calculated in this study. The values of S and C, calculated by using these different barriers, given in Table VII, indicate that the values of the thermodynamic functions, as reported in Table VI1 and in ref 11,differ by the contribution of the methyl torsional barriers. As previous studies of 1-butene2and 2-methyl1-butene4 have yielded a good agreement between the calorimetric thermodynamic functions and the statistical functions calculated by using methyl potential function barriers, we feel that the values of the thermodynamic
functions given in Table VI are probably correct. The calculated concentration of the gauche conformer, XB, 59.9% at 298 K, agrees well with that reported in the microwave study! at least 50% at 298 K, and explains the predominance of polarized bands in the Raman spectrum of the gaseous phase.
Acknowledgment. The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-79-20763. D.J.G. gratefully acknowledges the help of Dr. S. D. Hudson during the methyl torsion potential function calculations and Dr. D. A. C. Compton for suggesting the project.
Formation of Adsorbed Oxygen and Its Reactivity with Ethylene over Silver Catalysts Shulchl Kagawa, * Masakazu Iwamoto, Hlroshl Morl, Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
and Tetsuro Seiyama Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Fukuoka 8 12, Japan (Received: July 3, 1980)
The temperature-programmed desorption of oxygen from silver catalysts showed a single peak in the range 413-573 K with a maximum around 503 K. This temperature was not affected by the differences in the methods of preparation of silver samples. The amount of adsorbed oxygen increased in proportion to the logarithm of the adsorption pressure. This oxygen adsorbate led to the formation of ethylene oxide and carbon dioxide by reaction with ethylene, which were desorbed at 298-393 and 373-573 K, respectively. Both the amounts of produced ethylene oxide and carbon dioxide showed a bell-shaped dependence on the concentration of adsorbed oxygen; that is, the excess amount of adsorbed oxygen inhibited the productions of both ethylene oxide and carbon dioxide. In a similar manner the adsorption of carbon dioxide or ethylene oxide on oxygen-preadsorbed silver was investigated. On the basis of the results obtained, it was concluded that ethylene and oxygen adsorb competitively on silver atoms on which a slightly positive charge had been induced by the neighbor oxygen adsorbates. The mechanism for the oxidation of ethylene to ethylene oxide is also suggested,including the reaction of molecular oxygen adsorbates with ethylene adsorbed linearly on a silver atom.
Introduction In efforts to clarify a comprehensive mechanism for the silver-catalyzed oxidation of ethylene to produce ethylene oxide, several models have been suggested as active species.‘I2 A lot of evidence has been presented demonstrating the existence of molecular oxygen species by using electron diffra~tion,~ EPR,4 IR,S FEM,G and isotopic exchange reaction.'^^ Atomic oxygen species have also been proposed on a silver surface by a variety of technique^.^^^^^^^ (1) Hucknall, D.J. “Selective Oxidation of Hydrocarbons”;Academic Press: London, 1974;p 6. (2)Kilty, P.A.; Sachtler, W. M. H. Catal. Rev. 1974,10, 1. (3)Vol. Yu, Ts.;Shishakov, N. A. Izu. Akad. Nauk SSSR, Otd. Khim. Nauk 1962,586;Izu. Akad. Nauk. SSSR Ser. Khim. 1963,1920.Kagawa, S.;Tokunaga, H.; Seiyama, T. Kogyo Kagaku Zasshi 1968,71,775. (4)Clarkson, R. B.; McClellan, S. J. Phys. Chem. 1978, 82, 294. Abou-Kais. A.: Jarioui, M.: Verine. J. C.: Gravelle, P. C. J. Catal. 1977, 47,399. Tanaka, 8.; Yamashina, T. Ibid. 1975,40,140. Shimizu, N.; Jpn. 1973,46,2929. Shimokoshi, K.; Yasumori, I. BulZ. Chem. SOC. (5)Kilty, P. A,; Rol, N. C.; Sachtler,W. M. H. “Catalysis”;Hightower, J. W., Ed.; North Holland Publishing Co.: Amsterdam, 1973;p 929. (6)Czanderna, A. W.; Frank, 0.; Schnidt, W. A. Surf. Sci. 1973,38, 129. (7)Meisenheimer, R. G.; Ritchie, A. W.; Schissler, D. 0.;Stevenson, D. P.; Voge, H. H.; Wilson, J. N. Proc. Int. Congr. Surf. Act., 2nd,1957, 1957,2,337. Malgolis, L. Y. Izu. Akad. Nauk SSSR 1959,225. (8)Kagawa, S.; Iwamoto, M.; Morita, S.; Seiyama, T., unpublished results. (9)Kagawa, S.;Kono, K.; Futata, H.; Seiyama, T. Kogyo Kagaku Zasshi 1971, 74, 819. Wachs, I. E.;Kelemen, S. R. Proc. Int. Congr. Catal., 7th 1980,Preprint A-48. 0022-3654181l2085-0434$01.0010
At the present, however, many disagreements exist concerning the oxygen species active for the catalytic oxidation and the reaction mechanism. Considerable studies have suggested a mechanism in which the molecular oxygen species reacts with ethylene to give epoxide and the atomic species leads to formation of carbon dioxide and water.l-1° Based on this mechanism, a model for the intermediate giving ethylene oxide was proposed by Kilty and S a ~ h t l e rto~ be , ~ ethylene adsorbed symmetrically on the top of adsorbed molecular oxygen. However, Cant and Hallll claimed that such an intermediate forms carbon dioxide as well as ethylene oxide. Force and BelP2proposed an intermediate containing only one oxygen atom which is common to both epoxide formation and combustion. Recently, Kuczkowski et al.13 have discussed a variety of possible mechanisms. In addition, an interesting kinetic study by Harriott et suggested competitive adsorption of ethylene and oxygen on a partially oxygenated silver surface. (10)Herzog, W. Ber. Bunsenges. Phys. Chem. 1970, 74,216. Carra,
S.;Forzatti, P. Catal. Reu. 1977,15,1.
(11) Cant, N. W.; Hall, W. K. J. Catal. 1978,52,81. (12)Force, E. L.; Bell, A. T. J. Catal. 1975,38, 440; 1975,40,356. (13)Larrabee, A. L.; Kuczkowski, R. L. J. Catal. 1978, 52, 72. Egashira, M.; Kuczkowski, R. L.; Cant, N.W. Ibid. 1980,65,297. (14)Klugherz, P.D.;Harriott, P. AIChE J. 1971,17,856.Metcalf, P. L.; Harriott, P.Ind. Eng. Chem. Process Des. Deu. 1972,11, 478.
0 1981 American Chemical Society
Formation of Adsorbed Oxygen and Its Reactivity
The Journal of Physical Chernistty, Vol. 85, No. 4, 1981 435
TABLE I: Reduction Conditions and Surface Areas of the Silver Samples Used reduction conditiond catalyst
starting material
AgI
Ag,Oa
&-I1
Ag,C03b
torr 76 300 76
100 760 760
temp, time, K h 373 573 373 553 433 473
12 3 39
59
24
surface area, m’/g
HELIUM
OUTLET+
GAS RESERVOIRS
TCD
COLUMNS
o,20
o.30 o.06
a Silver oxide was precipitated from silver nitrate solution with sodium hydroxide solution. Silver carbonate was precipitated from silver nitrate solution with sodium carbonate solution. Silver nitrate was a commercial The reduction was permaterial (guaranteed grade). formed with hydrogen through two steps; first was by the upper row and second was by the lower.
The uncertainty in the mechanism for the epoxidation of ethylene, especially in the reactivity of the oxygen species adsorbed on silver, motivated the present study. Adsorption behavior of oxygen over silver was investigated by the temperature programmed desorption (TPD) technique.15 If the respective adsorbates, molecular and atomic oxygens, are in given energy states, the increasing desorption probabilities with a rise in temperature were expected to result in two peaks in the desorption flux. In the present paper, unfortunately, we could not detect two distinguishable desorption peaks probably because of rapid coupling reactions of atomic oxygens to form molecular oxygens. Hence, adsorption and/or reaction of ethylene, ethylene oxide, and carbon dioxide were studied over oxygen-preadsorbed silver. The correlations were then examined between the amount of adsorbed oxygen and the distribution of reaction products or the amounts of gases adsorbed later. From the results obtained, the nature of adsorbed oxygen and the reaction mechanism of ethylene over a catalyst are discussed. Although a similar experiment was partly carried out by Spath,16ahis experiments were insufficient in terms of nonlinear and very rapid heating, the lack of analyses of desorbed materials, and so on.lGb
Experimental Section Silver samples were prepared from three kinds of silver compounds by reduction with hydrogen. To avoid confusion, we employed silver without any support as a sample. For TPD experiments, the powder was sieved and a fraction of 42-80 mesh was used. The reduction conditions and surface areas are summarized in Table I. The surface areas of silver samples underwent considerable changes with the repetition of the oxygen adsorption-desorption cycles and were finally “stabilized” after the sixth cycle. The values of surface areas listed in Table I were measured after such stabilization. Commercial helium gas (>99.995%) was used as carrier gas. To remove trace amounts of moisture and oxygen, it was passed through a 5A Molecular Sieve column cooled (15) Cvetanovie, R. J.; Amenomiya, Y. Adu. Catal. 1967, 17, 103. (16) (a) Spath, H. T. “Catalysis”;Hightower, J. W., Ed.; North Holland Publishing Co.: Amsterdam, 1973; p 945. (b) For example, the one at 393 K of two desorption peaks found by Spath following the adsorption of O2 would be attributable t o the desorption of C02,as pointed out by Sachtler (Sachtler, W. M. H. “Catalysis”;Hightower, J. W., Ed.; North Holland Publishing Co.: Amsterdam, 1973; p 956), since Spath did not analyze the desorbed gases and the temperature of 393 K reported for the 0 2 desorption by Spath is very close to that for the desorption of carbon dioxide (373-423 K)ascertained by Sachtler and us (this work).
MANOMETER
TRAP OUTLET TRAP +ANALYSIS
UNIT
4-k
TPD UNIT d--
Figure 1. Apparatus used in TPD experiments.
with liquid nitrogen. Oxygen (>99.9%), hydrogen (>99.9%), and ethylene (>99.9%) were also dried before use by passing through a cold trap at liquid-nitrogen temperature. Carbon dioxide was obtained from dry ice, and ethylene oxide was prepared by the reaction of ethylene chlorohydrin and anhydrous potassium fluoride, followed by distillation. The apparatus used in the TPD experiments is shown in Figure 1. This consisted basically of the thermal desorption unit and the analysis unit. Desorbed gas was monitored with a thermal conductivity detector (TCD). The amount of silver sample mounted was 20 g, which was fairly greater than that in another TPD experiment8since the sensitivity of the TCD was not as high as that of a mass spectrometer. A gas chromatographwas connected directly to the desorption system in order to analyze the desorbed gas. The column packings were the following: Porapak Q for the separation of carbon dioxide, methane and ethylene; Bentone 34 for ethylene oxide and acetaldehyde; and 5A Molecular Sieve for oxygen and carbon monoxide. After mounting into the reactor, the sample was repeatedly subjected to a series of routine TPD procedures comprising sample pretreatment, oxygen adsorption, and temperature-programmed desorption. In a pretreatment at 573 K, each sample was exposed to 300 torr (1torr = 133.3 Pa) of hydrogen for 1h, followed by evacuation for 1 h. Oxygen adsorption was carried out at a desired temperature and pressure for a prescribed time and then evacuated at 298 K for 20 min. Subsequently, a carrier gas was diverted to flow through the reactor at a rate of 30 cm3/min, and the programmed heating was started. The heating rate was 5 K/min unless specified otherwise. In the TPD experiments with ethylene, carbon dioxide, or ethylene oxide, each gas was adsorbed on the sample at 298 K for 30 min after the above pretreatment or oxygen adsorption procedure. The subsequent TPD measurement was the same as the above. Although a little amount of oxygen was still desorbed above 573 K, the heating was limited to 573 K to avoid a decrease in the surface area of the sample. The amount of gas desorbed up to 573 K is hereafter called the amount of desorbed oxygen.
Results Characterization of Oxygen Adsorption. Typical TPD chromatograms of oxygen adsorbed on three kinds of silver samples are shown in Figure 2. The desorbed gas was confirmed to be only oxygen by means of mass spectrometry. The amount and the temperature of oxygen desorption were reproducible within an experimental error of -10%. The peak areas of the desorption chromatogram, however, seem to vary considerably with the preparation methods. The amounts of oxygen desorbed per unit surface area were 0.31, 0.17, and 0.12 cm3/m2, re-
436
:/....;:-
The Journal of Physical Chetn/stty, Vol. 85,No. 4, 1981
Kagawa et al.
1.0
z w
Ag-I
Po2 (Torr) 10 100 600
0.10
2 3
a
mw;a ' m 0.05 z-
2 0
a
O
-
2
0
2
In Po
2
4
6
8
'
(Torr)
Figure 3. Adsorption isotherm of oxygen at 453 K over Ag-I.
573
373 473 TEMPERATURE ( K
573
Flgure 2. TPD chromatograms of oxygen adsorbed on Ag-I, -11, and -111 at 453 K and 300 torr for 1 h.
TABLE 11: Effect of the Adsorption Temperature and Time on Oxygen Adsorption over Ag-I adsorption of oxygen temp, K
time,
433
0.5 6 0.2 1 17 0.5 6
453 473
h
desorbed peak press., amount, temp, torr cm3 STP/gof Ag K
314 293 320 314 346 312 309
4.58X l o - ' 6.50 X lo-' 5.14X lo-' 6.32X l o - * 6.14X l o - ' 3.02X lo-' 3.21 X lo-'
503 505 504 510 505 506 505
spectively, for Ag-I, -11, and -111, suggesting that the variation of the peak areas was partly due to the change of surface areas. In all chromatograms the desorption of oxygen formed a well-defined single peak which began at ca. 413 K and passed a maximum around 503 K. These results indicate that the sample preparation affects only the relative amounts of desorption peaks and that the essential nature of oxygen desorption (or adsorption) such as the bond streqgth between oxygen and silver remains unaffected. This suggestion is in good agreement with the observations by Ekern and Czandernal' and Seiyama and co-workers.ls With these facts in mind, the samples Ag-I and -11were subsequently used in the experiments, since the amounts of desorbed oxygen from these were greater than that from Ag-111. The effect of the adsorption temperature and time on desorption of oxygen is summarized in Table 11. Clearly, oxygen adsorption at 453 K was in equilibrium upon adsorption of oxygen for 1 h; thus the adsorption time was standardized hereinafter to 1 h in most experiments. In addition, it can be seen that the equilibrium amount of adsorbed oxygen was larger at lower temperature. It is noteworthy that the temperature of peak maximum was almost constant regardless of the adsorption temperature or the adsorbed amount. This phenomenon was also observed when the adsorption pressure was varied. The adsorption isotherm at 453 K on Ag-I is illustrated in Figure 3. The ordinate at the right side is the suface coverage, 8, which represents the number of atoms of adsorbed oxygen per surface silver atom. The number of silver atoms per unit surface area was evaluated at 1.31 (17) Ekern, R. J.; Czanderna, A. W. J . Catal. 1977,46, 109. (18) Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. Bull. Chem. Sac. Jpn. 1978,51, 2765; J. Phys. Chem. 1978,82,2564.
TEMPERATURE ( K )
Flgure 4. TPD chromatogram following adsorption of ethylene at 298 K and 300 torr for 30 min on Ag-I oxygen-preadsorbed at 453 K and 100 torr for 1 h.
X 1019atoms/m2 by Scholten and otherslg by using the equilibrium surface plane distribution of 25% for (loo), 5% for (110), and 70% for the (111) plane.20 The relationship between the adsorption pressure (Po,, torr) and the amount of desorbed oxygen (VO,, cm3/g-Ag) could be derived as eq 1. This relation was also found in an ad-
Vo, = 8.53 x In 4.02P0, (1) sorption isotherm over Ag-11, though the coefficients were different from the above. The interpretation or explanation of eq 1 is very difficult at the present, because Vo, consists of the amounts of two kinds of oxygen adsorbates as mentioned later. Adsorption and Reaction of Ethylene over Silver. Ethylene adsorbed on a silver sample only when it had previously adsorbed oxygen; that is, ethylene adsorption did not occur over a silver sample without adsorbed oxygen, in agreement with the results of several w ~ r k e r s . ~ ~ ~ J ~ Figure 4 is an example of the TPD chromatogram obtained after ethylene adsorption on oxygen-preadsorbed silver (Ag-I). Unreacted oxygen was desorbed in the same temperature region and with the same peak shape as shown in Figure 2. This indicates that the adsorption state of oxygen unreacted was not influenced by ethylene adsorbed later. Unchanged ethylene was desorbed in the range 298-373 K. Desorptions of ethylene oxide and carbon 1973,28, 209. (20) Sundquist, B. E. Acta Metall. 1964, 12, 67. (21) Marcinknowsky, A. E.; Berty, J. M. J . Catal. 1973,29,494. Seo, M.; Sato, N. Denki Kagaku 1971,39,629.
The Journal of Physical Chemistty, Vol. 85, No. 4, 1981 437
Formation of Adsorbed Oxygen and Its Reactivity 9 0.4
0
0 0.015- f a vr
i
-
1.2
0.8
0
0
0
3 ( 6)
W
E0 0010-
-am2
m
8 8 0.005Iz
0
-0oo1
C2H4
A4 -,,m
o
o 0.02u
004
0.06
AMOUNT OF ADSORBED OXYGEN ( cm3 STPlg-Ag 1
Figure 6. TPD chromatogram following adsorption of carbon dioxide at 298 K and 130 torr for 30 min on Ag-I oxygen-preadsorbedat 453 K and 100 torr for 1 h.
Figure 5. Yield of the desorbed gases following ethylene adsorption at 298 K and 300 torr for 30 min on Ag-I oxygen-preadsorbedat 453 K. Part A shows the total amount of irreversibly adsorbed ethylene, which was calculated from the amounts of the respective components plotted in part 8.
dioxide, which were the reaction products between ethylene and preadsorbed oxygen, were observed at 298-393 and 373-573 K, respectively. A t the stage of evacuation of the gas phase at 298 K prior to the heating procedure, a considerable amount of ethylene oxide was detected; therefore, this was added to the amount measured in the TPD experiments to calculate the total amount of ethylene oxide formed. A quantitative analysis of water vapor was impossible, and the desorption behavior of water was disregarded in Figure 4. As mentioned above, it was revealed that ethylene was adsorbed only on the oxygen-preadsorbed silver and that a part of adsorbed ethylene led to the formation of ethylene oxide and carbon dioxide. It was further examined how the amounts of these reaction products varied with the amount of preadsorbed oxygen. The results are depicted in Figure 5, where the amount of preadsorbed oxygen was estimated from the oxygen adsorption pressure by using eq 1. The amounts of ethylene oxide and carbon dioxide increased initially with the amount of preadsorbed oxygen and reached maxima at 8 = 0.7 and 0.4, respectively. A similar phenomenon was also recognized in the case of Ag-11. It is worth noting that there are maxima in the formation of ethylene oxide and carbon dioxide depending on the concentration of adsorbed oxygen and that the value of 0 at the maximum of ethylene oxide formation is larger than that of carbon dioxide formation. In Figure 5, the amount of irreversibly adsorbed ethylene, which was calculated from the amounts of carbon dioxide, ethylene oxide, and ethylene desorbed at elevated temperatures, is also plotted. Adsorption of Carbon Dioxide and Ethylene Oxide over Silver. The adsorption of carbon dioxide was observed only on the oxygen-preadsorbed silver sample, in accordance with other work.22 A typical desorption chromatogram is shown in Figure 6, following adsorption of carbon dioxide after preadsorption of oxygen on silver. The chromatogram consisted of two peaks: the one at the lower temperature was carbon dioxide and the one at the higher (22) Czanderna, A. W. J. Colloid Interface Sci. 1966,22,482. Wydeven, T.;Leben, M. J. Chromatogr. Sci. 1969, 7,445.
c 2
s 3
0
0 0.02 0.04 0.06 AMOUNT OF ADSORBED OXYGEN ( c m3 STP1g-Ag)
Figure 7. Yield of desorbed carbon dioxide following carbon dioxide adsorption at 298 K and 130 torr for 30 min on Ag-I oxygen-preadsorbed at 453 K.
temperature was oxygen. The temperature at peak maximum of carbon dioxide was 383 K. This differs from the desorption temperature of carbon dioxide produced in the reaction of ethylene with adsorbed oxygen, which was 423 K, as indicated in Figure 4. This suggests that the adsorption state of the surface species desorbing as a carbon dioxide in Figure 4 was not the same as that of carbon dioxide in Figure 6. Coadsorption of water vapor did not influence the desorption behaviors of oxygen and carbon dioxide. This excludes the possibility that the presence of water might change the desorption temperatures of oxygen and carbon dioxide. Figure 7 shows the relationship between the amounts of preadsorbed oxygen, which was estimated in a similar manner with that in Figure 5, and adsorbed carbon dioxide. The adsorption of carbon dioxide was carried out at 298 K for 30 min in the presence of -130 torr of carbon dioxide. Under this condition the adsorption of carbon dioxide appeared to be at saturation, since there was no increment of desorbed amount upon the adsorption at higher pressure or for a longer period. As indicated clearly in Figure 7, the amount of adsorbed carbon dioxide increased linearly with the amount of preadsorbed oxygen except for the nonlinearity in the region of quite small amounts of oxygen. The dependence of the amount of
430
The Journal of Physical Chemistty, Vol. 85, No. 4, 1981
Kagawa et al.
e 0.4
0
s
0.8 t
I
P o 2
0.021 I
I
c
I /
\
I
u AMOUNT OF ADSORBED OXYGEN ( cm3 STP1g-Ag 1
Figure 8. Yield of the desorbed gases followlng ethylene oxide adsorption at 298 K and 100 torr for 30 min on A g I oxygen-preadsorbed at 453 K.
3
; 0; 0.02 0.04 006 AMOUNT OF ADSORBED OXYGEN ( c m3 STP1g-Ag)
Flgure 9. Correlation between the amounts of oxygen preadsorbed and consumed in the reaction with ethylene at 298 K. Broken line indicates one-to-one stoichiometry between them.
adsorbed carbon dioxide on the concentration of preadsorbed oxygen is different from that in Figure 5. This demonstrates the difference between the adsorption sites for carbon dioxide and ethylene. Finally, the adsorption of ethylene oxide was studied. Ethylene oxide was desorbed below 393 K, which was identical with the desorption temperature of ethylene oxide formed in the reaction of ethylene with adsorbed oxygen. Carbon dioxide was formed from the adsorbed ethylene oxide, and it was desorbed around 463 K. This temperature differs from that of carbon dioxide desorption following ethylene adsorption or carbon dioxide adsorption on oxygen-preadsorbed silver. This disagreementindicates the difference between the precursors for carbon dioxide from ethylene and ethylene oxide. The correlation between the amount of preadsorbed oxygen and that of adsorbed ethylene oxide is illustrated in Figure 8. The relationship was similar to that for ethylene (Figure 5) except for the values at zero preadsorbed oxygen, suggesting that the adsorption site for ethylene oxide is the same as that for ethylene.
shown in Figure 9 as a function of the amount of adsorbed oxygen. The broken line in the figure was drawn on the assumption of one-to-one stoichiometry between the adsorbed and consumed amounts of oxygen. It is characteristic that the consumption of oxygen was greater than the adsorbed amount in the range of small values. This is due to the existence of sorbed oxygen other than that found in the present work. On the basis of the other works?17* it is evident that a fair amount of oxygen exists on or in silver, which cannot be removed from the sample by hydrogen reduction at a temperature as high as 623 K. This oxygen species would contribute to the deviation of the consumption of oxygen from the one-to-one stoichiometry at the small amounts of adsorbed oxygen. This oxygen would also cause the nonlinearity in the relation between the amounts of adsorbed carbon dioxide and preadsorbed oxygen in Figure 7. Reaction Mechanism for Ethylene Oxidation. The variation of the amount of adsorbed ethylene showed a bell-shaped dependence on the concentration of adsorbed oxygen in Figure 5. That of ethylene oxide has a very Discussion similar behavior, as shown in Figure 8. However, the amount of adsorbed carbon dioxide increased monotoniOxygen Species on Silver. A maximum desorption rate cally with the amount of adsorbed oxygen. It is concluded of oxygen has been observed in the range 423-573 K upon that the adsorption of oxygen results in the formation of oxygen adsorption at 423 K by Czanderna and co-workat least two kinds of adsorption sites on silver samples, one ers.17 R ~ v i d has a~~ reported that the desorption of oxygen of which is useful for carbon dioxide adsorption and the adsorbed on the (110) plane of silver single crystal reached other is available for ethylene and ethylene oxide ada maximum at 553 K. These results, including those by sorption. In addition, the decrease in the amount of adTokora et alaz4and Kagawa et ala8are consistent with the sorbed ethylene or ethylene oxide with an increase in the results in the present work; however, the oxygen adsorbate amount of adsorbed oxygen would result from coadsorption suggested in each investigation is different. As indicated of oxygen onto the latter site mentioned above. Therefore, in our separate paper,8the oxygen species corresponding the latter site is concluded to accept all of the adsorption to the desorption peak in Figure 2 are a mixture of moof ethylene, ethylene oxide, and oxygen. The following lecular and atomic oxygen. This is well supported by the reactions for adsorption of several gases are suggested: result in Figure 3; i.e., the value of surface coverage above unity indicates the presence of molecular oxygen species. 2Ag + O&) Ag-02 S (44 It must be emphasized that the value was calculated from + 02(g) 2Ag-0 + 2 s (4b) 4Ag the amount of desorbed oxygen, not including absorbed oxygen. The atomic oxygen species is dominant at lower (5) Ag-02 + Ag e 2Ag-0 surface coverage, while the molecular form increases in S + C2H4 S-CZHI (6a) concentration with increasing surface c o ~ e r a g e . ~ * ~ ? ~ ~ The amount of oxygen consumed in Figure 5 was S C2H40 S-C2H40 (6b) evaluated by eq 2 and 3. The oxygen consumption is s 0 2 s-02 (64 C2H4 + 302 2C02 + 2H20 (2) Ag-0 + C02 Ag-CO, (7) Molecular and atomic oxygen can be proposed as species adsorbing on the silver surface, any electronic charge on
--
-
+
+
-
+
-
+
(23) Rovida, G. J. Phys. Chem. 1976, 80, 150. (24) Tokoro, Y.; Uchijima, T.; Yoneda, Y. J. Catal. 1979, 56, 110. (25) Czanderna, A. W. J. Phys. Chem. 1964, 68,2765.
(26) Sandler, Y. L.; Durigon, D. D. J. Phys. Chem. 1965, 69, 4201.
The Journal of Physical Chemistry, Vol. 85, No. 4, 1981 430
Formation of Adsorbed Oxygen and Its Reactivity
the species being neglected in the above equations. Adsorption of such oxygen species causes the formation of another kind of adsorption site which is designated as S. Site S may be a silver atom adjacent to the silver atom on which an oxygen species is adsorbed. This site S is available for adsorption of ethylene, etc., through eq 6a-c. Kilty and Sachtler2 have suggested the following intermediate for ethylene oxide formation:
reaction scheme for the epoxidation reaction on silver: -
0
I
0
I
I 0 CH27CH2
I+
-Ag-Ag A8 -
d -Ag-Ag-
A9
However, this model cannot explain the reason for the behavior that the amount of adsorbed ethylene decreased with an increasing amount of preadsorbed oxygen. In the above mechanism, eq 4a-6c, it can be explained by competitive adsorption between oxygen and ethylene. This suggestion is supported by the observation of Harriott et al. in the kinetic studyI4 where the formation rates of ethylene oxide and carbon dioxide showed maxima in their dependence on the partial pressure of oxygen. On the other hand, carbon dioxide is adsorbed on the Ag-0 species as an carbonate ion by reaction 7. The adsorption of carbon dioxide as an carbonate ion has been recognized on many oxides or metal^.^' Next, the reaction of ethylene with adsorbed oxygen will be considered. It should be remembered that the maximum of ethylene oxide formation was shifted to higher oxygen coverage than that of carbon dioxide in Figure 5. Based on the discussion in the former section, it is reasonable to conclude that the molecular and atomic oxygen species lead to the formation of ethylene oxide and carbon dioxide, respectively (eq 8a-9). These reactions are similar S-CZH4
+ 6Ag-0
- + -+
+
S-C2H40
-
+ Ag-0 2H20 + 7Ag
S-C2H40
2C02
S
C2H40
@a)
’\ p
-Ag-Ag
O-CH,
I
S-C2H4 + Ag-02
-
IS+
6
)CHp
2
-
-
-
0
I+
-Ag-Ag-
a+
+C2H40
Here the adsorption site S corresponds to @+.Habgood% and Carter et al.B found the adsorption of ethylene on Ag+ exchanged in X zeolite. The latter group reported that there were two kinds of ethylene adsorbates, one of which was desorbed upon evacuation at 373 K and another was not removed upon evacuation at 473 K. The former was estimated to rotate relatively easily around the C-C axis. These observations support the above scheme that the site S is Agb+and that ethylene coordinates linearly on the silver atom, since the adsorption of ethylene found in this work apppears to correspond to the former reported by Carter et alaz9The mechanism including the linear adsorption of ethylene is also borne out by the stereochemical study using microwave spectroscopy. Several g r o ~ p s l l J ~ ~ ~ have examined the rotation around the Cz axis upon the oxidation of ethylene-1,2-d2to ethylene oxide over various silver catalysts and concluded that the rotation is appreciably free, in agreement with the mechanism suggested in the present work. The retention of configuration in ethylene oxide may depend upon the lifetime of ethylene coordinating linearly on a silver atom. It is worthy of remark that Ozinsl has suggested the following surface intermediate on the basis of matrix chemistry:
(8b)
(9)
to the reactions suggested by many workersl-10except for the situation of ethylene. In the above mechanism, both epoxide formation and combustion proceed through the Langmuir-Hinshelwood mechanism in which chemisorbed ethylene molecules react with oxygen adsorbates. There is no direct physicochemicalevidence for the structure of intermediate, but at the present we postulate the following (27) For example, Little, L. H. “Infrared Spectra of Adsorbed Species”; Academic Press: London, 1966; Chapter 3.
The intermediate for the formation of carbon dioxide is unknown yet. It is clear that the intermediate from ethylene to carbon dioxide is different from that from ethylene oxide to carbon dioxide or the adsorption state of carbon dioxide itself. (28) Habgood, H. W. Can. J. Chern. 1964,42, 2340. (29) Carter, J. L.; Yates, D. J. C.; Lucchesi, P. J.; Elliott, J. J.; Kevorkian, V. J. Phys. Chern. 1966, 70, 1126. (30) Richey, W. F. J. Phys. Chern. 1972, 76, 213. (31) Ozin, G. A. Acc. Chern. Res. 1977, 10,21.
