1667
HYDROGENAT~ON AND SELF-HYDROGENATION OF ETHYLENE
Initial Hydrogenation Activity of Tungsten. Hydrogenation and Self-Hydrogenation of Ethylenelapb by Robert R. Rye and Robert S. Hansen Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 60010 (Received October 18, 1968)
Ultrahigh vacuum techniques have been combined with rapid scan mass spectroscopy to study the initial hydrogenation and self-hydrogenationreactions of ethylene on a clean tungsten surface. Admission of ethylene to a hydrogen-covered surface at 300"K results in extremely rapid conversion into ethane in an amount corresponding to approximately 0.1 of an ethylene monolayer. Admission of ethylene to a clean tungsten surface again results in rapid conversion into ethane but is characterized by a short induction period during which only ethylene is observed in the gas phase. Using the maximum hydrogen coverage resulting from surface dehydrogenation during this induction period, order of magnitude calculations are used to show that the initial rapid selfhydrogenation is not reasonable in terms of a Rideal-Eley mechanism involving gaseous ethylene and hydrogen on paired sites but is reasonable in terms of a Langmuir-Hinshelwood mechanism involving mobile hydrogen and immobile associatively adsorbed ethylene. Tungsten appears to have an appreciable activity for catalytic hydrogenation in the range 160-200'K. The lower limit is established by the mobility of hydrogen and the upper limit by the stability of the associative form of ethylene. At a minimum temperature of 195°K where flash decomposition studies2have shown ethylene to be stable, the surface does not poison.
Introduction With the development of ultrahigh vacuum (UHV) techniques and rapid scan mass spectrometers it has become possible to follow continuously the gas-phase composition during the adsorption of the first hydrocarbon monolayer on a metal surface. This region for the hydrogenation of olefins has not been tractable previously due to the extreme rapidity of the initial reaction. Immeasurably fast initial hydrogenation reactions have been observed previouslya and form the main experimental basis for the Rideal-Eley-type reactions involving the collision of a gas-phase ethylene molecule with adsorbed hydrogen. The use of gas pressures in the Torr range seriously complicates the study of this initial reaction, for according to the kinetic theory of gases the rate of arrival of gas molecules per square centimeter at a surface is dnldt = (2amlcT)-'/'P (1) where m is the mass of a molecule, Tis the gas temperature, and P is the gas pressure. At a pressure of 1 Torr the rate of arrival suffices, with unit sticking coefficient, to form a monolayer in -1 psec. If the resulting hydrogenation or self-hydrogenation reaction is fast, the initial reaction would be essentially instantaneous. If, however, the pressure in eq 1 is decreased, the time required to form the initial monolayer is correspondingly increased. I n the present study the base pressure was 1 X 10-lo Torr with the sample gas leaking into a final pressure of Torr. With these conditions the time required to form the first hydrocarbon monolayer has been extended to as long as 2 min. Through the use of a rapid scan mass spectrometer and
a high-speed recording system, the initial reaction can be easily observed in this time period. These conditions, UHV and low gas pressure, have been used previously in decomposition studies of hydrocarbons a t metal surfaces by means of flash filament s p e c t r o s ~ o p y ~and ~ ~ field emission microscopy.5J Roberts'J has used UHV-deposited thin films to study the decomposition of saturated hydrocarbons on a number of clean metal films and the self-hydrogenation of ethylene on iridium. I n Roberts' experiments, however, high gas pressures ( =40p ) were employed, and the initial self-hydrogenation reaction was too fast to follow.
Experimental Section The experimental system used in this work was similar to that used for flash filament decomposition studies.2 Only those modifications in the apparatus and techniques needed for hydrogenation studies will be (1) (a) Based in part on a dissertation submitted by R. R. Rye (1968) to the Graduate College of Iowa State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (b) Work was performed in the Ames Laboratory of the Atomic Energy Commission; Contribution No. 2410. (2) R. R. Rye and R. 8. Hansen, "Flash Decomposition of Acetylene, Ethylene, Ethane and Methane on Tungsten," J . Chem. Phys., in press. (3) (a) 0. Beeck, Discusaione Faraday Soc., 8 , 118 (1950); (b) G. I. Jenkins and E. Rideal, J . Chem. Soc., 2490 (1955). (4) R. S. Hansen, J. R. Arthur, V. J. Mimeault, and R. R. Rye, J . Phys. Chem., 70,2787 (1966). ( 5 ) J. R. Arthur and R. S. Hansen, J . Chem. Phys., 36, 2062 (1962). (6) N. C. Gardner and R. 6. Hansen, "A Field Emission Study of R s actions of Ethylene and Acetylene on Tungsten," t o be published. (7) R. W. Roberts, Brit. J . Appl. Phys., 14,485 (1963). (8) R. W. Roberts, J . Phys. Chem., 67,2035 (1963).
Volume 78, Number 6 June 1060
1668
ROBERT R. RYEAND ROBERTS. HANSEN
50 Mil TUNGSTEN TO IONlZATl GAUGE
TO MASS SPECTROMETER
&TO
GAS SUPPLY
TO
PUMPS
Although the system contained both an ionization gauge (Westinghouse 5966) and the mass spectrometer, all data were taken with the mass spectrometer which contains a thoria-coated iridium filament capable of low operating temperatures. The ionization gauge was, however, used to establish a relation between gauge pressure and mass spectrometer output. With a steadystate pressure of ethylene established in a highly pumped system the ionization gauge indicated a nitrogen equivalent pressure in Torr ranging from three to seven times the mass 28 ion current in amperes from the mass spectrometer. The ionization gauge calibration factor, X, which relates the observed positive ion current, I+, to the emission current, I-, and the pressure, p , by
I + = I-XP
Figure 1. Sketch of the experimental cell containing both a ribbon (lower portion of cell) and a filament (center of cell).
discussed here. The experimental cell, shown in Figure 1, was rebuilt to include both a filament and a ribbon with geometric surface areas of 0.8 and 8 cm2, respectively. Both a ribbon and a filament can be used in the same apparatus since tungsten is compIetely inactive for the initial fast hydrogenation reaction unless previously cleaned. The temperature of either could be controlled independently, the filament by conduction through the use of coolants in the small dewar and the ribbon by surrounding the lower portion of the cell with a dewar containing a suitable coolant. In the former case only the filament is cooled with the walls remaining at room temperature. The mass spectrometer was connected to the central cell by 19 cm of tubing 2.54 cm in diameter giving a conductance of 11 l./sec9 between cell and mass spectrometer. The mass spectrometer was a Nier type, 90” sector instrument capable of both electrostatic and magnet scans, and is similar to the one developed by Davis and Vanderslice.lo In the electrostatic mode of operation scan rates of >lo0 amu/sec are possible. The detector end of the mass spectrometer consists of an electron multiplier with a gain of -lo6 resulting in an output ion current in amperes of the same order as the pressure in Torr. This output ion current was amplified using a Keithley Model 415 electrometer having a rise time of 1 msec, and the resultant amplified signal was recorded on a direct writing oscillographic recorder with full-scale frequency response of 50 cps. For these rapid mass scans, the mass range from 24 to 31 amu, which contains the major mass peaks of acetylene, ethylene, and ethane, was chosen. l1 Since a mass 30 peak is unique to ethane, and acetylene has no mass peaks greater than a small isotope peak a t mass 27, the experimental mass 28 and mass 30 ion currents can be used to determine the percentages of ethylene and ethane. The Journal of Physical Chemistry
(2) is a function of both gauge geometry and gas composit i ~ n . The ~ ratio of gauge factors is, however, independent of gauge g e ~ m e t r y . ~Using the gauge factors reported by S c h ~ l zand ’ ~ by Santeler, et aZ.,13the nitrogen equivalent pressure was converted into hydrocarbon pressure with the result that to a fair approximation (to a factor of three) the mass 28 ion current in amperes is equal to the pressure in Torr, The cell was first pumped to a base pressure of a t least 2 X 10-’0 Torr, and either the filament or ribbon was cleaned of adsorbed material by resistance heating to i= 3000°K. Previous field emission results6 have shown that this procedure produces a clean surface. The sample was allowed to cool for 2 min to reach the base temperature, and the ground-glass valve separating cell and pump was closed, limiting the pumping speed to less than 0.5 l./sec. Hereafter the system with ground-glass valve closed will be referred to as “closed,” although the small pumping speed remaining permitted maintenance of desired ambients at steadystate pressures by suitable adjustment of leak rate. For hydrogenation experiments a pressure of 1 X 10+ Torr of hydrogen was first established in the closed system, and the Granville-Phillips leak valve used to control the ethylene pressure which increased to a final pressure of approximately 3 X 10-6 Torr over a time interval ranging from 30 sec to 2 min. Recording of the 24-31 amu range was started when the first increase in hydrocarbon pressure was detected in the Torr range and continued until the reaction ceased. For (9) 6. Dushman, “Scientific Foundations of Vacuum Technique,” J. M. Lafferty, Ed., 2nd ed, John Wiley & Sons, Inc., New York, N. Y.,1962. (10) W. D. Davis and T. A. Vanderslice, Transactions of the Seventh National Vacuum Symposium (1960),1961,p 417. (11).American Petroleum Institute, Mass Spectral Data: Research Project 44, Washington, D. C., National Bureau of Standards, 1949-1959. (12) G. J. Schulz, J. A p p l . Phws., 28,1149 (1957). (13) D. J. Santeler, D. J. Holkeboer, D. W. Jones, and F. Pagano, “Vacuum Technology and Space Simulation,” National Aeronautics and Space Administration, Washington, D. C.,1967.
1669
HYDROGENATION AND SELF-HYDROGENATION OF ETHYLENE HYDROGENATION
self-hydrogenation experiments the procedure was the same except that no hydrogen was present. The ethylene used in these experiments was Phillips research grade with a purity of 99.98 mol %. The hydrogen was admitted to the system and at the same time purified by diffusion through a palladium tube. XIO~
Results Figure 2 contains the results of a hydrogenation experiment in which over 2 min was required for the hydrocarbon pressure to reach the 10-6 Torr range. The experimental cracking patterns a t 62 and 63 sec are given in Table I along with the cracking pattern of ethane reported in the American Petroleum Institute tables. l1 The agreement between the experimental and API crack\ing patterns shows that in this region the ambient is essentially ethane. The main body of Figure 2 contains a plot of mass 30 and 28 ion currents extending over five decades vs. time. The time axis refers to the time after the first increase was detected in the hydrocarbon peaks in the 10-lo Torr range. Although ethylene was admitted )o the filament predosedowith hydrogen, the first recorded hydrocarbon ambient was >85% ethane. Table I : Experimental Cracking Patterns Calculated from Data in Figure 2 Inset and the API Cracking Pattern for Ethane Mass
62 Bec
63 8eo
API tables11
31 30 29 28 27 26 25
2.0 26.4 22.5 100 36.2 26.9 4.8
2.2 25.2 22.0 100 36.2 27.5 5.5
0.5 26.2 21.7 100 33.3 23.0 4.2
Using the cracking pattern of ethane, the ethane contribution to the experimental mass 28 ion current can be calculated from the observed mass 30 ion current. The ethylene contribution to the observed mass 28 ion current can then be obtained by difference. In this manner the ion current data in Figure 2 were converted into ethylene and ethane partial pressures; the results are shown in Figure 3. Until a hydrocarbon pressure of -1 X Torr is reached the hydrocarbon ambient is composed chiefly of ethane. Between 0 and 50 sec an ethylene pressure can be calculated; however, the amount is small (less than 10% PQHJwith a great deal of scatter in the points. Between 50 and 100 sec no ethylene can be detected. Ethane production is rapid until a pressure corresponding to about 3.9 X 1013 molecules of ethane is reached; thereafter, it substantially ceases and the pressure begins to drop as the ethane is slowly pumped off. Continued admission of ethylene after this results only in an increase in the ethylene partial pressure. Assuming two-point attach-
k
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l
a
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'
0
9
XIO~
I
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I
IO
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50
70
90
110
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I50
170
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Figure 2. Mass spectrometer ion currents obtained during the initial rapid hydrogenation reaction a t 300°K. The inset contains & small segment, covering the interval from 61 to 65 sec, of the experimental mass scans. From such data the mass 28 and mass 30 ion currents were extracted and plotted v.s. time in the body of the figure.
ment for ethylene, the ethane produced corresponds to the hydrogenation of -0.1 of an ethylene monolayer. At 300°K the initial hydrogenation reaction on a clean surface predosed with hydrogen is extremely rapid with the upper limit for the rate of hydrogenation apparently near the rate of arrival of ethylene molecules a t the surface. The reaction, however, also rapidly poisons. After the initial fast reaction, if the valve to the pumps was opened to remove the ambient and then closed, the subsequent increase in hydrocarbon ambient was composed entirely of ethylene. The rapid hydrogenation at 300°K is limited to the first hydrocarbon monolayer. If the filament was not cleaned of adsorbed species (predominantly CO if no prior hydrocarbon experiments were conducted) or if the filament was held at 95"K, no hydrogenation activity was observed a t any time. These experiments served the additional purpose of eliminating the possibility of reactions occurring on portions of the system other than the filament or ribbon. In the 95°K experiments, only the filament was cooled with the walls being a t room temperature. From flash decomposition studies2 the dehydrogenation of surface ethylene is known to have an appreciable rate a t 200°K suggesting that the self-poisoning of tungsten results from the dehydrogenation of chemisorbed ethylene to chemisorbed acetylene. For this reason the hydrogenation reaction was conducted on the Volume 79, Number 6 June 196s
1670
ROBERT R. RYEAND ROBERT S. HANSEN I
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tungsten ribbon a t a minimum temperature of 195°K obtained by surrounding the lower portion of the cell with powdered Dry Ice. Initial fast hydrogenation similar to the reaction a t 30OOK was obtained, but without the associated self-poisoning. By repetitive opening and closing of the valve to the pumps, this hydrogenation was conducted for as long as 2 hr with no noticeable diminution of catalytic activity. If the ethylene is admitted to a clean filament a t 300°K in the absence of hydrogen, rapid self-hydrogenation occurs. The ethylene and ethane partial pressures obtained during the initial self-hydrogenation are shown in Figure 4. As hydrogen was not introduced as such, it must have been furnished from the decomposition of ethylene. Thus there is an induction period associated with this reaction; no ethane is observed until the pressure reaches 1 X low8Torr a t 18 sec. The amount of ethane is, of course, lower than in the hydrogenation reaction. Only 5.8 X 10l2 molecules are produced which, again assuming a two-point attachment of ethylene, corresponds to the hydrogena5 an ethylene monolayer. tion of ~ 0 . 0 1 of The induction period permits simple calculation of the approximate surface coverage a t the point where self-hydrogenation becomes appreciable. For this purpose ethylene pressure was replotted on a linear scale in Figure 5 from zero time to 18 sec where self-hydrogenation is observed. Assuming a sticking coefficient of 1, the integrated form of eq 1 can be used to calculate the number of ethylene molecules adsorbed during this interval. If a two-point attachment of ethylene is assumed, the surface coverage of ethylene is given by
The Journal of Physical Chemistry
Figure 4. Ethylene and ethane partial pressures obtained during the initial rapid self-hydrogenation reaction on tungsten a t 300°K.
where NW is the surface density of tungsten sites, 1 X 10l6 atoms/cm2. The integral in eq 3 was obtained graphically from Figure 5 , and the ethylene coverage a t 18 sec was found to be approximately 0.04 monolayer.
Discussion Fast initial reactions such as this have been observed in the past on tungsten and other metals3r8and have been used as experimental evidence for the Rideal-Eleytype mechanisms involving a reaction between gaseous ethylene and adsorbed hydrogen. Beeclr interpreted this fast initial reaction as occurring between gaseous ethylene and chemisorbed hydrogen on adjacent sites but pointed out that since ethylene competes strongly with hydrogen for surface sites, eventually a certain rate of hydrogenation of chemisorbed ethylene will be necessary to maintain hydrogen sites for impact hydrogenation. The self-hydrogenation reaction rate observed in this work is too fast to be supported by the Beeck variant of the Rideal-Eley mechanism. The maximum rate for such a mechanism occurs if every ethylene molecule impacting a pair of sites both containing hydrogen atoms is essentially reflected as an ethane molecule. The rate of impact per square centimeter is given by eq 1, and if is the fraction of sites occupied by hydrogen atoms, the probability of impacting two sites both occupied by hydrogen atoms is at most OH2. Hence, ignoring any activation energy or orientation effects (which could only decrease the rate) the maximum number of ethane molecules which can be produced per
HYDROGENATION AND SELF-HYDROGENATION OF ETHYLENE
ing chemisorbed ethylene to produce ethane (Jenl&sRidealab variant of a Rideal-Eley mechanism) is even less than that supportable by the Beeck variant. Consider next a Langmuir-Hinshelwood-type mechanism involving mobile chemisorbed hydr0gen~~8~5 and immobile associatively adsorbed ethylene.616 Presumably this mechanism involves the following reaction sequence16 * * kl * CH2 - 6H2 H CH2 - CH3 (6)
Pdt=4,8xlQ8torr sec
j-001 90-
ao70-
60-
4
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1671
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t I
*
&ON40-
CH2
j
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IO
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18
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Consider the maximum rate supportable by reaction 6, assuming the hydrogen to behave as a mobile twodimensional gas. The number of collisions per square centimeter per second, 2 is
( 1 1 1 1 1 1 1 1 1 1 ( 2
- CHa +
k -1
* k2 H Jr C&L(g)
22
=
t (sed
Figure 5. Change in ethylene pressure from time zero when ethylene is first observed in the gas phase until self-hydrogenation is observed a t 18 sec. Graphical Torr sec. integration of this curve yields a value of 4.8 X
square centimeter per second by this mechanism is given by
(4)
'//~~H~ENw~VU
(8)
where v is the speed of the mobile hydrogen, u is the collision diameter, and OH and BE are hydrogen and ethylene surface coverages, respectively. The actual maximum rate is less than this for two reasons. First, the hydrogen adatoms are not completely mobile but have an activation energy AHd for diffusion, and second, there is an activation energy AH* for the reaction, Accounting for these factors, the maximum rate of production of chemisorbed ethyl radicals by reaction 6 is
If the filament has an area A and the system a volume V , the maximum rate of ethane pressure increase is given by (9)
The factors in eq 5 can be completely evaluated a t the point where self-hydrogenation is proceeding a t an appreciable rate. The ethylene pressure a t this point is 1 X IO-* Torr. From the results obtained in Figure 5, the maximum ethylene coverage a t this point is 0.04 monolayer. If we assume surface dehydrogenation yielding chemisorbed acetylene and hydrogen to be complete a t 300°K (reasonable in light of flash decomposition studies2), the maximum hydrogen coverage, OH, is expected to be a t most 0.04 monolayer. I n the present system A = 0.8 cm2 (geometric area), V = 1 1. in eq 5 , and the maximum rate is = 2 X 10-l'J Torr/sec. The observed maximum rate (Figure 4) is greater than this by at least an order of magnitude. It therefore cannot be supported by ethylene molecules impacting surface hydrogen atoms (Beeclr3" variant of a Rideal-Eley mechanism) even if there is no activation energy for the reaction. The ambient hydrogen partial pressure in this experiment did not exceed 2 X 10-lO Torr. An argument similar to the foregoing shows that the maximum rate supportable by hydrogen molecules impact-
where v is the jump frequency and d is the jump distance. If Beeck's3" value of 2.4 kcal/mol for the activation energy for hydrogenation and Gomer's16 value of 9.5 kcal/mol for the activation energy of hydrogen migration a t moderate coverages are used, eq 9 can be simplified, using the followinog reasonable values : Ny = 1 X 10l6sites/cm2, d = 2 A, Y = 1013sec-', u = 4 A, T = 300°K to d (6H2-CH%) = 0 dt
~
x01019~
(10)
Further evaluation of this requires knowledge of the extent of dehydrogenation; however, if we assume that even 99% of the ethylene coverage of 0.04 monolayer a t the point where self-hydrogenation is appreciable has dehydrogenated according to reaction 5, the rate (14) W. J. M.Rootsaert, L. L. van Reijen, and W. M. H. Sachtler, J . Catal., 1,416 (1962). (15) R. Gomer, R. Wortman, and R. Lundy, J . Chem. Phye., 26, 1147 (1967). (16) J. Horiuti and M. Polanyi, Trans. F a r a d a ~ SOC., 30, 1164 (1934). Volume 73, Number 6
June 1969
1672
ROBERTR. RYEAND ROBERTS. HANSEN
Figure 6. Mass 30 ion current (proportionalto ethane pressure) obtained during the flash decomposition of an associative ethylene monolayer farmed at 95%.
by eq 9 is 1 X IO" ethyl radicals/(cm' sec) or 0.2 monolayer/sec. Only a tenth of this number would be needed to maintain the observed rate. A similar calculation.indicates that eq 7 would alsompport the observed rate. The Langmnir-Hinshelwood-type mechanism, set forth by eq 6 and 7 involving mobile hydrogen and immobile associatively adsorbed ethylene, is therefore consistent with the observed maximum rate. This contention is supported by the self-hydrogenation obtained as a minor product during the flash decomposition' of a low-temperature ethylene monolayer. Figure 6 contains the mass 30 spectrum (the mass 30 ion current is directly proportional to the ethane pressure) obtained during such a flash. The production of ethane has a detectable rate a t = 170°K where hydrogen is reported to have a detectable mobility on tungsten"," and reaches a maximum rate a t 250'K where decomposition is extensive.' After formation of the ethylene monolayer at 95°K where adsorption is associative,' this self-hydrogenation is independent of the hydrogen pressure and essentially independent of the ethylene pressure. The lower limit for hydrogenation appears to result from an activation energy for the mobility of hydrogen and not an activation energy for the hydrogenation step. The same lower limit can be inferred from Kemball's description of his low-temperature deuteration experiments." The rapid self-poisoning of tungsten a t 300'K results from rapid surface dehydrogenation. Below 200°K where this reaction is not appreciable,' the surface does not poison. This self-poisoning is highly irreversible; no hydrogenation of acetylene was observed even when it was dosed on to a surface saturated with hydrogen a t 300'K. As previously indicated, a similar treatment of ethylene led to its rapid hydrogenation. Kemball" measured the rates of deuteration of ethylene on tungsten andrhodium a t 173"K, and Beeck** the rates Of hydrogenation Of ethylene On the same metals at 273°K. Comparing these ratios, Kemball
-'
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,
18dsorbPd acelylene
Figure 7. Cork ball models depicting the strain-free configurationsof chemisorbed ethylene and acetylene imposed by the surface topography. trans-Ethylene would occur on spacings >2.89 b, while &-ethylene would occur on spacings
