T H E DECOMPOSITIOS OF ACETONE I N COST.1CT WITH PLATIS’URI BY H. AUSTIN TAYLOR
The coniparisons that have so far been made between homogeneous and catalysed reactions are confined to those reactions that are of second order in the gas phase, becoming in general unimolecular when suitably catalysed. The function of the catalyst seems to be to allow a series of simple reactions to occur, the energy of activation of any single step being less than that of the homogeneous reaction. The problem naturally arises: what effect will a catalyst have on a truly homogeneous unimolecular reaction? I n an attempt to solve this problem the reaction chosen was the decomposition of acetone which Hinshelwood and Hutchinson’ have shown to be truly unimolecular. There appears to be some possible doubt about the actual reaction which was studied in this case, namely whether, as the authors claim, it is the primary split into carbon monoxide and free methyl groups, or whether it is actually the decomposition of ketene. Whatever the true answer may be, suffice that here evidence is given that it is the same reaction being studied in both cases. That acetone is readily decomposed catalytically has been shown by Sabatier.* “The decomposition occurs slowly at 240’ and rapidly a t 270’ in presence of nickel, yielding carbon monoxide and the CHI radicals which give a little ethane and ethylene but chiefly methane, hydrogen and carbon.” It is the latter substance, carbon, that precludes the use of nickel as a catalyst for kinetic measurements. Several attempts were actually made to follow the reaction in this manner. I t was found possible to obtain a single velocity curve with a freshly prepared catalyst3 but the deposition of an apparently abnormal amount of carbon even a t zooo almost completly poisoned the catalyst for succeeding runs, so the method was discontinued The action of platinum sponge and palladium black on ketones is stated by Sabatier to be less intense. It seemed likely therefore that the bulk metal would be somewhat active though perhaps only feebly. This less intense action might reduce the quantity of carbon formed, since Hinshelwood found only a slight blackening of the reaction vessel in the homogeneous decomposition. The poisoning action would consequently also be reduced. The method adopted therefore was to study the decomposition on a hot platinum filament stretched axially across the reaction vessel which was kept a t a constant temperature of soo in a thermostat. The reaction was followed by the pressure change with an apparatus similar to that used by Hinshelwood, ‘Proc. Roy. Soc., 111A, 24j (1926). Ann. Chim. Phgs., (8) 4, 474 (1905). 3 The catalyst vas nickel reduced from the nitrate supported on pumice.
1iY4
H. AUSTIS TAYLOR
the manometer and capillary connecting tube being aound nith fine nichronw aire and maintained as near to 50' as practicable to maintain the acetonr gaseous during a run Acetone, purified by the standard bisulphite iiiethod and then twice di-tilled, was finally distilled into a small flask sealed to the reaction ves-el through a three-way stopcock The apparatus nas evacuated by meany of an oil pump for at least thirty minutes between runs, the filament being
TABLE I A . Temperature
=
8440c'
Initial Pressure
t?.
t-,
7 0
18.4
7.4 6.8
16.6
53
6.4
15.2
2-52
i .O 6 . (1
1:
8.6 8 6
21
6
22
0
3 93 315 302 2
23 i 226
16.i
B. Temperature
=
.o
I;._.
890Y'
382 33'
C'.
1T.1
2 4 2 .i
7 . 2
27
6 .O
'7.5
6.0 5.4
I2 j
2 io
* .1
268
2 .I
229.. i
2
182.j
1.9
4.6
I
Temperaturr 413 360 303,i 237.5 I73
=
2
-
*
3 ' J
0
15.0
6 9.8
12
91ho(' 3 5
17
1.1
3.1
Ij.0
1.1 1.1;
2.9 2 4
8.7 6.0
0.8
2 3
6.2
1
'
.o
allowed to glow during the latter part of the evacuation. This was found necessary to avoid the slight induction period which is apparent in some of the results at the low temperaturr, and may have been due to adsorbed hidrocarbons or possibly carbon itself on thr filainmt. In order t o maintain the temperature of the filamrnt constant throughout a run, and avoid temperature changes due to the changing thernial conductance of the surroundings, its resistance was kept constant by placing it in one arm of a Wheatstone Bridge, the standard resistance being of 3 ohms and capable of carrying I j amps. The current, actually consumed bring only about I . j amps. the heating effect in the latter, and con>ryuently the change in resistance \vas negligible. The 'bridge' itself consisted of twI I 0,ooo ohiii 1-ariableresistance
DECOMPOSITION O F ACETOKE IS COSTBCT WITH PLATINUM
320r
FIG. I
FIG.2
1796
H. AKSTIS TAYLOR
boxes so that the current carried by them was also negligible. A delicate galvanometer was used for the balance, which was maintained by varying the external current through a rheostat. That some such arrangement was necessary is shown by the fact that the current consumed during runs varied by 0 . 2 amp. between the beginning and end. The actual temperature of the filament was obtained with an optical pyrometer, an auxiliary object lens being used the better to focus the filament. These temperatures were checked by the resistance of the filament, a small correction due to the cold ends being necessary. Measurements were made at three different temperatures using varying initial pressures of acetone. The somewhat surprising result was that in all cases the course of the reaction was quite similar to that found by Hinshelwood and susceptible t o t,he same analytical treatment. The degree of concordance between successive runs does not appear to be quite so good as in the homogeneous reaction, which may probably be accounted for by the changing catalytic effect on one or other of the reactions occurring. That the reaction is unimolecular may be judged from Table I which gives the times taken for the initial pressure to increase by z j, 50 and jj per cent, The results of t Z 5and tjj are ample evidence that the reaction is essentially unimolecular. The variations in the values of t , j are no doubt due to the subsequent reactions that are possible after the initial decomposition has occurred. That such reactions when taken into consideration would better the agreement between the various values at t Z 5and at tsa as well, can be seen in the following examples of t25corrected for B in Table I. The values become 1.5, 1.6, 1.6, 1.6, 1.6, 1.5 respectively.' There can be no doubt therefore as to the order of the reaction in general. The results obtained for the rate of pressure increase with time are shown in the following diagrams, one for each temperature as stated. The curves are identical in form with those in the homogeneous reaction being unimolecular in character up to approximately i o per cent of the change but from that point on continuing to increase slomly but steadily, instead of reaching a maximum value. Hinshelxood has suggested this t o be due to the subsequent reactions of the free methyl groups, in which case the limiting slope of the observed curve would correspond to the pressure increase accompanying those reactims. By extrapolating back to zero time as in Fig. 4, the value of the pressure increase is obtained that would have been observed had the primary decomposition of the acetone alone occurred. This value is the value to n-hich the pressure increase becomes asymptotic for the simple unimolecular decomposition. The observed pressure increases after different time intervals are therefore greater by an amount which is the difference between the above asymptotic ralue and the limiting slope of the ~~
~
The value 2 . 2 above must not he compared with the 1.6 here given since the former is the time taken for the initial pressure t o increase by 25 per cent., whereas the latter i s d c u l a t e d on the basis of the primary decomposition alone, as shown later.
DECOMPOSITIOS OF ACETONE IN CONTACT WITH PLATINUJl
FIG.3
/ / M E
IN
FIlN.5.
FIG.4
I797
I798
H. A U S T I S TAYLOR
observed curve at each particular time. It is therefore a simple matter t o construct the ideal unimolecular curve in each c a v , the lower curve in Fig. 4 being one example.
TABLE I1 Time in mins.
Cqrr. pressure
Ob;. pressure increase in mms. 0
4
21
18
8
39 51
38
hi i8
60
8; 96
i6 82
16 20
21 28
2.303 log
t
Y
0
I2
-
increase 0
49
69
32
102
87
36
lo;
91
40
112
94
44 48
11;
96
,0583
120
52
I22
98 99
0594 . o 584
56 60
I25
IO0
.0.;82
I 28
IO1
.0j91
64 68
131
I02
I34
103
,0618 ,0683 Arean=
,oji
TABLE 111
-4. Temperature
=
844’ C . Corrected Final Pressure
Initial Pressure
Nean k
220
0.063
104
0.05;
382
230
0.208
33 1 268 2 73
2
229.:
162
0 . I97
.j
J3h
0.228
302
165 13. Tempcraturc
182
C’. Trnipcrature
=
890°C.
= 9I 6O(
18
0,196
I80
0.208
*.
413
270
360
216 214 191 I30
3 0 3 :. 237 5 173
0.202
192
‘04 104-5
DECOJIPOSITIOS O F .ICETOSE I S C O S T A C T WITH PLATISUhI
1799
From the values so obtained graphically it is easy to calculate the unimolecular constant at each temperature. Table I1 gives the results obtained in one case. The initial pressure was 1 6 j mms. and the temperature 844'c'. The extrapolated value of the pressure increase is 104 nuns. and the unimolecular constant is calculated on that basis as shown.
The degree of constancy is quite good when consideration is taken of the fact that a small error in the limiting slope of the observed curve reflects itself considerably in the constants calculated. Table 111 summarizes the average constants calculated for various runs made. That the reaction so studied, and giving the above velocity constants is substantially the same as that observed by Hinshelwood, is evident from the fact that the percentage increase in pressure (corrected) in both cases is of the same order. Hinshelwood cites two cases showing percentage increases of j 7 and 79 respectively at different temperatures. The variation.
I 800
H. AUSTIN TAYLOR
here noted are greater than this but show no definite trend either with change of pressure at a particular temperature or with change of temperature. The average increases for example are j j, 68 and j z percent in sets A, B and C respectively. It seems reasonable to assume then that the corrected pressure increases are due to the same decomposition as in the homogeneous study. This is further borne out by the calculation of the heat of activation from the velocity constants at the three temperatures Plotting the logarithms of the velocity constants against the reciprocals of the absolute temperatures the straight line in Fig j is obtained, The slope of this line corresponds t o 68,400 calories which is also the average of the values calculated by the Arrhenius equation for the temperature intervals 844-890, 890-916 and 844-916'C. This value is an excellent confirmation of the value 68,500 calories given by Hinshelwood. That the value is not a fictitious one introduced by the particular method of mathematical analysis of the observed results is shown by the accompanying plots on the same figure which are the logarithms of the average values of the previously mentioned times for 2 j, j o and 7 5 per cent increases of the initial pressure (and therefore uncorrected) plotted against the reciprocals of the absolute temperature. The t z j and tm values obviously lie on a straight line whose slope is almost identical with that for the velocity constants. The values for t;5 show a variation which in view of the foregoing is to be expected. The general conclusion to be drawn from the results therefore would appear to be that the primary decomposition takes place in the gaseous phase in the hot zone immediately surrounding the platinum filament, the latter merely serving as a source of heat and in no way acting catalytically. That the platinum may affect the succeeding reactions is possible. The particular method of analysis would not permit a definite decision on the point although certain irregularities in the later part of the reaction might be taken to suggest a small disturbing effect in the reactions of the methyl groups when compared with the homogeneous case. The observation by Langmuir' that carbon monoxide is held to platinum excessively strongly may account in part for the small catalytic effect, in that the surface is practically completely poisoned. It could not however preclude the possibility of activation of a molecule in the immediate neighborhood of the wire by radiation or by actual collision with the poisoned filament. It would seem peculiar that a spontaneous decomposition of the acetone molecule should occur in preference to a bimolecular reaction such as that found by Allen and Hinshelwoodz in the case of acetaldehyde. The facts presented however prove quite definitely the unimolecular nature of the reaction under all the conditions studied and the identity of the heat of activation herein found, with that for the homogeneous reaction is strong evidence for an identical mechanism. I n so far therefore as the original intention of the paper is concerned, to investigate the catalytic decomposition as compared with the homo'Trans. Farada) SOC., 17, 621 (1922). * Proc. Roy. Soc., 121A, 141 (1928).
DECOMPOSITIOS O F A C E T O S E IS C O S T A C T K I T H PLATISCX
I801
geneous, the work has failed. The point mentioned in an earlier portion h o w ever seems worth emphasizing at this stage namely that under other conditions, as for example with an active nickel catalyst, the deposition of carbon is far in excess of that observed during the above work. This would suggest that under the latter conditions the actual decomposition is different from that presumed above. Such a difference might be solely in the subsequent reactions of the methyl group although the possibility exists that the initial decomposition might vary in the two cases as well It would appear possible then, that under those conditions where a catalytic effect is to be observed the results would not be directly comparable with those for the homogeneous reaction and so the initial object of the work defeated. h decision on the point would require further investigation.
Summary The decomposition of acetone was studied at various temperatures and pressures in contact with a heated platinum filament. 2. The results are identical in form with those for the homogeneous reaction, the same method of mathematical analysis being applicable. 3 . The primary decomposition is unimolecular, with an energy of activation of 68,400 calories as compared with 68, joo calories for the homogeneous case, suggesting a homogeneous reaction. in a zone around the hot filament, the latter showing no catalytic effect. 4. The large amount of carbon deposited during the decomposition on active nickel presents the possibility of a different, mechanism which may not be comparable with the homogeneous change. I.
.Vichols Chemical Laboratory,
.Yew Y O Tr ~n i m s i t y , .Ye% I-ork, S. I-,
