Y. AMIESOMIYA AND E. J. C V E T l N O V I C
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Vol. 67
APPLICATION OF FLASH-DESORPTION METHOD TO CA4TALYSTSTUDIES. I. ETHYLENE-ALUMINA SYSTEM' BY Y . AMENOMIYA AND R. J. CVETANOVIC Division of Applied Chemistry, National Research Council, Ottawa, Canada Received July 6, 1969 The flash-desorption technique was applied t o the ethylene-alumina system to investigate the surface of alumina. The apparatus consisted of a programming controller increasing the catalyst temperature linearly with time a t various speeds and of a thermistor type thermal conductivity cell for measuring the rate of desorption of ethylene. The existence of two different sites for the adsorption of ethylene on alumina was established. On one of these sites the activation energy of desorption was 26.8 kcal./mole (I), and on the other 36.4 kcal./mole (11). These sites occupy 2.8% of the total surface area, 60% of which belongs to I, 40% t o 11.
Introduction Flash-desorption technique has been used by several investigators2-8 to obtain kinetic information on adsorption and desorption of gases on metals. Very interesting information recently was obtained wit'h this technique by Redhead,* who studied desorption of gases from a metal filament using ultrahigh vacuum techniques. The desorption was followed by measuring pressure changes by means of an ionization gage. The filament, temperature was increased linearly with time and the activat'ion energy of desorption was obtained by assuming the temperature independent factor in the Arrhenius equation to be 10'3. The present work has been carried out under conditions much more similar to those ordinarily employed in cat,alytic reactions. Desorption of ethylene has been studied in an attempt t.o obtain information on the properties of the cat,alyst surface of alumina. A thermistor type thermal conductivity cell was used to detect the ethylene desorbed from the catalyst in a stream of helium. The speed of increase of catalyst temperature was variable and considerably slower than used by previous authors. Experimental Apparatus.-The apparatus used is shown in Fig. 1. The reactor, R, is a quartz tube of 8-mm. i d . provided with a perforated disk on which a small amount of alumina is placed. The part of the apparatus to the right of stopcocks SI and SI is an ordinary static system which can be used to measure the adsorption when SIand SZ are closed. The amount of ethylene adsorbed on the catalyst was measured in the usual way using the manometer, M, and a cathetometer. Following this, the catalyst was evacuated and then the stream of helium was admitted through SI,over the catalyst in R, through 52, the thermal conductivity cell, D, and finally through the liquid nitrogen trap, Tq. Helium from the cylinder was passed through silica gel a t liquid nitrogen temperature to remove traces of water. The flow rate of helium was measured by a soap film flowmeter attached t o the outlet of the rotary pump. When the catalyst tpmperature is raised a t a uniform rate hy means of the programming controller, ethylene desorbs from the catalyst and is carried with helium past the thermal conductivity cell, D, by means of which the concentration of ethylene in helium is determined in the same manner as in gas chromatography and is recorded on a recorder (Leeds & Northrup Speedomax, full scale: 1 my., response time: 1 sec.). Since the flow rat'e of helium is constant, the deflection of the recorder due t o the pres-
__
(1) Contribution no. 7142 from the National Research Council, Ottawa, Canada. (2) G. Ehrlich, J . Phys. Chem., 60, 1388 (1956). (3) (a) T. W.Hiclcmott and G. Ehrlich, J . Chem. Phys., 24, 1263 (1956): (h) J. Eisinger, ibid., 27, 1208 (1967). (4) R. E. Schlier, J . A p p l . Phys., 29, 1162 (1958). (5) T. W. Hiokniott and G. Ehrlich, J . Phys. Chem. Solids, 5, 47 (1968). (6) G. Ehrlich, J . A p p l . Phys., 32, 4 (1961). (7) G. Ehrlich, J . Chem. Phys., 34, 29, 39 (1961). (8) P. A. Redhead, Trans. Faraday Soc.. 57, 641 (1961).
ence of ethylene in the carrier should be proportional t o the rate of its desorption. The rate of desorption increases with temperature a t first but eventually begins t o decrease as a result of depletion of adsorbed gas so that a peak is recorded. The recorded peaks will be called in the present paper the flash-desorption chromatogram. Simultaneously, the temperature of the catalyst is recorded by means of a second recorder which is connected t o an Alumel-Chrome1 thermocouple, TCZ,inserted directly into the center of catalyst bed, as shown in the inset of Fig. 1. As the charts of the two recorders are driven a t the same specd, the temperature a t any point of the flash-desorption chromatogram can be determined. The temperature a t which peak maximum appears is related to the activation energy of desorption, as will be discussed later. The surface area of the thermocouple, TC2, and of its leads was so small that their effect on the adsorption was neglected. The temperature indicated by the inside thermocouple TC2 was always a little higher than that indicated by TCI. The temperatures reported here are those read by means of TCe. The speed of raising the catalyst temperature could be varied from0.5 to4O0/min. The detector used was a conventional thermal conductivity cell of thermistor type, AEL 9677 of the Gow-Mac Instrument Co. Materials.-Alumina was prepared by adding an excess of a 10% ammonium hydroxide solution to a 20% aluminum nitrate solution a t room temperature. The precipitate was washed with distilled water several times,* dried at l l O o , and crushed to small cubes of about 2 mm. The weight of alumina placed into the reactor was 0.253 g. and the lengtli of the catalyst bed was 13 mm. Before use, alumina was treated with air for 2 hr. a t 600' and evacuated a t the 6ame temperature for more than 60 hr., until no more water condensed in the trap TI during evacuation. After each run the catalyst was evacuated for 2 hr. a t 600' and this was followed by evacuation for 12 hr. at 550". The total BET surface area of the catalyst after this treatment was 41.7 m.2 for the sample of 0.253 g. used. The amount of ethylene adsorbed in each run under the same conditions was found t o be constant within the experimental error, indicating that the above pretreatment was adequate for the cleaning of the catalyst surface. Phillips research grade ethylene was passed through activated charcoal, condensed in a liquid nitrogen trap, and evacuated. A trace impurity of ethane was present but no attempt was made at further purification. Procedure.-In every run the adsorption of ethylene was first measured. When ethylene was admitted to the catalyst, a pressure change was observable only within 2 or 3 min.; the amounts adsorbed reported here all were measured after 10 min. After this measurement the catalyst was evacuated for 10 min. a t room temperature (series I), and then exposed t o the helium flow.. When the recorder base line of the thermistor detector became stable after several minutes, indicating that no gas was evolved from the catalyst, the flash-desorption was started. In the experiment series 11, the catalyst was heated up quickly t o 100" in vucuo after the adsorption measurement and evacuated for 60 min. a t the same temperature instead of the evacuation for 10 min. a t room temperature in series I. The subsequent procedure was the same as in series I. The amount of gas desorbed by flash-derorption can be mcasured by either (1) trapping in the trap TI followed by gas chromntographic analysis, or ( 2 ) measuring the arca of the Amh(9) The final p H of the wash water was between 8 and 9. carried out for residual nitrate.
No analysis was
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APPLICATION O F I'ILASH-I)EGORPTION METHOD TO CllTALYsT STUDIES
40.
30 0
Fig. 1.-Diagram
300
400
500 TPC)
of the apparatus.
desorption chromatogram. The latter method contained, however, some uncertainty due t o the shift of the base line during flashing, although this agreed with the result of the former method within the experimental error of about 10%. The values reported in this paper all were obtained by the former method.
Results and Discussion Figure 2(a) shows the typical flash-desorption chromatogram of an experiment in series I, Le., with the ca,talyst evacuated for 10 min. a t room temperature before the flash-desorption. Figure 2(b) was obtained at a different speed of flashing. The speed of raising temperature is given in the figures as 6 ('C./mEm.). It is seen that two superposed peaks are obtained. The results of series I are summarized in Table .[ in which T M lis the temperature at which the first peak maximum appeared. The amount of gas desorbed during the A ashing was measured as described above. When the catalyst was evacuated for 60 min. at 100" before the flash-desorption (series II), the iirst peak disappeared and only the second peak was left as seen in Fig. 2(c). This peak can be well superposed onto the second peak of Fig. 2(b) obtained at approximately the same p, as shown in Fig. 2(d). The results of series I1 are listed in Table I1 in which Tal2 is the temperature at which the second peak maximum appeared. T M 2 values taken from the chromatograms of series I were slightly different from those listed in Table I1 due to the influence of superposition of the first peak upon the second peak. In similar experiments with ethane instead of ethylene, no peaks such as with ethylene were observed. If ethylene was introduced to the catalyst under higher pressure than 50 mm. for a long time, the main peak of the flash-desorption chromatogram appeared a t higher temperature and the gas chromatographic analysis showed that the desorbed material consisted almost entirely of butenes. However, the present experiments were done at pressures below 10 mm. and both peaks in series I and I1 were identified by gas chromatography as ethylene containing less than 0.5y0 of C, and a trace of C4hydrocarbons. When the temperature, T, of the catalyst is raised, the following general relation should hold dz6/d12 = d26/bt2
+ (d2$/btbT+ dW/bTdt)dT/dt + b20/bT2(dT/dt)2 + be/bl'.dzT/dt2
where 6 is the surface coverage and tis the time.
20
IO
Id0
2bO
360
4bo
30
t (min.1
500
TPC)
I
I
C 0.4
i E
20 I
100
200
I.o
30
360 4bO
do0
t(min.1
T ("CI
-EXP 34
& 0
100
I
1
200
300
460
5/00
C'
Fig. 2.-Flash-desorption chromatograms. (a) Expt. no. 22: evacuated before flashing 10 min. at room temperature; @ = 10.OSO/min. (b) Expt. no. 29: evacuated before flashing 10 min. a t room temp., p = 16.03"/min. (c) Expt. no. 34: evacuated before flashing 60 min. at looo, @ = 15.90°/min. (d) Superposition of chromatograms from Expt. no. 29 and 34.
If the temperature is increased linearly with time, so that T = Td Pt, where T and To are, respectively, the temperature at time t and the initial temperature, and if it is assumed that M / d t >> dB/dT throughout
+
Y . AMENORIIYA AND R. J. CVETANOVIC
146
I
5.0
I
I
TABLE I ADSORPTION AiYD FLASH-DESORPTION O F ETHYLENE.SERIES I (Alumina evacuated for 10 min. a t room temperature before flash-desorption, flow-rate of He 45.7 f 0.3 cc./min.)
I
Adsorption
c
Expt. no.
t
1
3.5
Fig. 3.-Plot
of eq. 3 for series I.
the course of an e~periment,~ the above equation can be simplified to d26/di2 = b2e/bt2f pb26/dT&
A t the same time the rate of desorption, Td, -uo,d8/dt
=
(1) is written as
koO exp(-Ed/RT)
(2) where 1c0 is a constant, Ed is the activation energy of desorption, and um is the amount of gas adsorbed a t 0 = 1. From (1) and ( 2 ) , and putting dzO/'dtz = 0 a t T = T M , we have rd =
Ed/?V,n/RT&f2ko= exp(-Ed/RTd which is the same as Redhead's result,* or 2 log Tiis - log /3 = Ed/2.303 RTM
T'ol. 67
+ log EdVfn/Rko
Temp.,
Pressure, mm.
OC.
22 24.9 23 24.8 24 24.7 26 25.2 27 26.2 28 26.5 29 25.8 44" 24.0 45 24.8 46 24.2 50 25.0 "Flow-rate of gage.
.
7-Flash-Desorption-Amt. of gas Amt. of gas adsorbed, p, T M I , desorbed, cc. ( N T P ) "CJmin. OC. CC. (NTP)
6.13 6.34 6.37 5.28 5.63 5.38 5.96 5.73 1.43 9.00 0.002b He, 102.5
0.218 .227 .239 ,273 ,259 .251 ,266 ,323 .212 .344 .097 cc./min.
10.08 74 ... 20.70 79 ... 5.05 68 ... 5.14 72 ... 21.05 81 ... 10.40 74 0.091 16.03 77 ,095 16.35 77 16.1C .. ,102 16.05 78 ,105 15.78 .. ,099 'Measured by hlcLeod
where f* and fa are the partition functions of the activated complex (excluding k T / h v ) and adsorbed molecule, respectively, c, is the surface concentration of the adsorbed molecule, and k and h are t'he Boltzmann and Planck constants, respectively. Also assuming t'hat the freedom of both the activated complex and the adsorbed molecules is so small that f* fa 1, we have
+ +
Td =
c,(kT/h) exp(-&/RT)
where kT/h corresponds to ko/vm in eq. 2. The calculated value of k T / h a t 35OoK is 0.73 X 1013 sec.-l and is in reasonable agreement with the observed Jc0/~, = 1.6 X lo1&sec.-l considering that the freedom of t.he activated complex might be greater than t.hat of adsorbed molecules, L e . , .f*/fa > 1. Unfortunat,ely, a good linear plot as in Fig. 3 was not obtained for the second peak (series 11). In this case the activation energy of desorption was calculated from eq. 3 assuming that ko/um was 1.6 X 1OI6 as obt'ained for the first peak. The values of E d are given in the last column of Table I1 and the mean value is 36.4 kcal./mole. Since the adsorption reached equilibrium very quickly (usually within a few minutes),
(3) The plot of the left hand side of eq. 3 against l / T Mgives, therefore, a straight line, from which E d and ko/Vm are obtained. TABLE I1 The assumption that >> &/bT appears ADSORPTION AND FLASH-DESORPTION O F ETHYLENE.SERIES 11 reasonable because in a fast flow of helium the rate of (Alumina evacuated for GO min. a t 100" before flash-desorption; readsorption should be negligible. If the rate of readflow-rate of He 45.7 f 0.3 cc./min.) sorption had an effect, Tw and the shape of the peak --Adsorption-----. -Flash-Desorptionwould be altered by a change in the ffow rate of helium Actisimilarly to the changes occurring in adsorption gas vation Amt. of h m t . of energy chromatography. However, expt. no. 44 in Table I, gas adgas deof dewhich was done a t more than twice the flow rate than Pressorbed, 6, sorbed, sorption, in the remaining experiments, gave the same T M and E x p t . Temp., sure, cc. "C./ Tm, 00. kca1.l no. mm. (NTP) min. "C. (NTP) mole peak shape. In addition, the good linearity of the 33 26.1 5.77 0,278 17.00 202 0.044 36.6 plot of eq. 3 also supports the above assumption, as 34 25.9 5.61 ,285 15.90 203 .045 36.8 will be shown later. 6.33 .307 19.80 20i .044 36.8 In Fig. 3, 2 log T M - log p was plotted against l / T ~ u 35 25.4 36 26.0 5.87 .311 9.87 186 ,041 35.8 for the first peak (Table I). From the slope and 38 26.0 5.72 .289 31.95 210 ,041 36.6 intercept E d and ko/vm were found to be 26.8 kcal./ 39 25.5 6.05 ,306 40.32 208 1 36.3 mole and 1.6 x 10'5 sec.-l, respectively. Assuming 42 26.0 5.76 ,305 16.00 194 36.0 that the adsorption of ethylene on alumina is non,041 36.5 47 25.0 2.03 ,214 15.55 200 dissociative and localized, the absolute rate of desorp48 25.0 2.22 ,234 15.80 203 ,042 36.7 tion is written as 49 25.0 10.11 .377 15.50 195 ,039 36.1 OC.
rd
= ca(lcT/h)(f*/fa) exp(---d/RT)
Mean 36.4
AP~LICATION O F FLASH- DIZSORPTION
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147
AIETHOD TO CATALYST STUDIES
z
Ql
Q
! i
LT
E:D
a
I
0 AMOUNTC ;eH, D E S O R B E D BY F L A S t ESORPTION (SERIES I V A C U A T I O N AT ROOM T E M P E R A T U R E 1
rp--w0
2
4
o-
AMOUNTOF C l H + DESORBED B Y F L A S H DESORPTION ( S E R I E S 1 1 . EVACUATION AT l0O0C I
6 8 IO 12 14 PRESSURE OF ETHYLENE (mm).
16
18
Fig. 4. --Isotherm of ethylene at room temperature.
the observed activation energies of desorption will be approximately equal to the heats of adsorption. I n Fig. 4 the amount of gas desorbed by flashing and the amount of adsorbed gas are plotted against the pressure at which the adsorption was measured. It is seen that the amount of the gas desorbed is constant and independent of the pressure a t which adsorption was carried out. This indicates that the strong adsorption (chemisorption) which is not removed by evacuation at room temperature is already saturated a t a pressure as low as 0.002 mm. There exist, therefore, two types of chemisorption of ethylene on alumina: one corresponds to 0.058 cc./0.253 g. of alumina with a heat of adsorption of 26.8 kcal./mole, the other to 0.042 cc./0.253 g. with a heat of adsorption of 36.4 kcal./ mole. The BET area of this catalyst was 41.7 m.?/ 0.253 g., as mentioned before. v, was found to be 3.64 cc./0.253 g. from the Langmuir plot ranging froim 30 to 150 mm. of ethylene. Combining this urn with BET area, 43 A.z is obtained as the sectional area of an
0.5
I .o
43 Fig. 5.-Change
of the heat of adsorption with surface coverage (ref. 10).
adsorbed molecule. Although this value seems to be a little too large, it is of the right order of magnitude. If 3.64 cc. is adopted as urn, 0.058 and 0.042 cc. correspond to surface coverages of 1.6 and 1.2%, respectively. If an adsorbed molecule experiences the repulsive force from a molecule adsorbed on an adjacent site and not from other molecules, the heat of adsorption varies with the surface coverage as shown in Fig. 5, so that two distinctly different heats of adsorption may be observed on homogeneous surfaces.lo I n the present case, however, the distanoce between chemisorbed molecules is a t least about 29 A , , provided ethylene molecules are dispersed homogeneously on the surface. This distance is too large for the repulsive forces between molecules to be effective. Therefore it is unlikely that the existence of two different heats of adsorption depends on the interaction between adsorbed molecules except if the active sites on which ethylene chemisorbs are dispersed gatchmise on the surface. The shapes of the two peaks on the flash-desorption chromatograms were broader than calculated assuming a constant E d . In view of this, it may be possible that a heterogeneity of the surface is involved. This matter, however, cannot be discussed a t present. (10) K. J. Laidler, “Catalysis” (P. H. Emmett), Vol. 1, Chap. 3, Reinhold Publ. Gorp., New York, N. Y., 1954.
