NOTES
March 20, 1953
*
The entropy at 298.16'K. is 18.13 0.03 e.u. of which 0.03 e.u. was contributed by the extrapolation below 15°K. Acknowledgment.-We wish to acknowledge D. Wood who assisted in the assistance of Mr. UT. the calculations. CRYOGENIC LABORATORY AND DEPT. OF CHEMISTRY THEOHIO STATE UNIVERSITY COLUMBUS, OHIO
Catalytic Activity of Lanthanum and Strontium Manganite BY G. PARRAVAXO RECEIVED JUNE 30, 1952
Recent work of Jonker and van Santenl and Volger2 has shown that compounds of the general formula (La,-, Sr,) (Mn?, MnP)O3 can be prepared from a mixture of the corresponding oxides or carbonates. Strontium up to 6 = 0.7 can be introduced in the LaMnOa phase without changing its perowskite structure. These compounds have a magnetic Curie temperature which is dependent on the amount of strontium added, and the tempera-
1497
ture coefficient of the electrical resistivity shows anomalous, behavior; it is positive for the ferromagnetic state and negative above the Curie point. It was therefore considered interesting to investigate the possibility of a relationship between the anomalous behavior of the temperature coefficient of the resistivity of these compounds and the activity of the solid surface to catalyze a chemical reaction. The compound Lao.&r~.a&tnOa was chosen for this investigation because i t possesses the highest Curie point (-373°K.) of the series, and because i t was found that a t that temperature the oxidation of carbon monoxide on the above compound proceeded a t rates which could be experimentally detected. La~.~aSr~.raMnOl was prepared by ball milling a mixture of stoichiometric amounts of C.P. manganese carbonate, strontium oxide and lanthanum oxalate for 1 hour. The mixture was then fired in a platinum crucible in air a t 850" for 5 hours, milled again for 1 hour and finally heated a t 900' for an additional 3 hours. Purification of gases and apparatus used for the catalytic experiments have already been described . 3 It was found that the experimental data on the oxidation reaction could be fitted quite well to a first-order equation (Figs. 1, 2). Values for the velocity constant, k(min.-', g-l), a t different temperatures are presented in the following table and in Fig. 3, where they are plotted according to the Arrhenius equation, from which an over-all activation energy of 10.6 kcal./mole could be derived. TABLE I OXIDATIONOF CARBONMONOXIDEON Lao.ssSro.rsMnOa 1, o c .
CO/& k X 108, r n h - 1 , g-I
50 81.5
400 800 1200 1600 2000 2400 2800 3200 Minutes. Fig. 1.-Oxidation of carbon monoxide on Lao.&o.sbMnO, (2.139 g.): CO/Oz = 2/1; PO = 280 mm.: V, 50'; 0 , 81.5'; 8 , 88'; A, 96'; @, 100"; 0, 105'.
..
0
h
88 95 100
0.34 1.45 1.37 1.69 2.17
-
2/1, PO t,
-
OC.
105 140 183 212
280 mm.
x 101, mm. -1, g -1
k.
3.83 11.8 49.0 82.0
16 14 12
4 10 $ 8 \
21 22 23
24
25 26 27 28 29 30 31 x 104. Fig. 3 . 4 x i d a t i o n of carbon monoxide on Lao.&ro.a6MnOa. Arrhenius plot.
I/T
8'6 1 4 2
r(
100 150 200 0 , O 100 200 300 400 500 A Minutes. Fig. 2.-Oxidation of carbon monoxide on Lao.&3ro.ssMnOr (2.139g.): CO/Os = 2/1; PO = 280 mm.: A, 140'; 0, 1830; e , 212O.
From the experimental data it is clear that the oxidation reaction shows the same activation energy below and above the Curie point of the catalyst. This is in disagreement with the magneto-catalytic effect studied by H e d ~ a l l . According ~ to this effect the temperature coefficient of the activity of a catalyst is markedly different below and above its
(1) G. H.Jonker and J. H. van Santen, P k y s i c a , 16,337 (lQ50). (2) "Semiconducting Materials," Butterworth Scientific Publications, London, 1951, Chapter b y J. Volger.
(3) G. Parravano, THIS JOURNAL, 74, 1448 (1952). (4) For a review on this effect see: Schwab, "Handhuch der Katalyse," Vol. 6, Springer, Verlag Wien, 1943,chapter by J. A. Hedvhll.
50
Curie point. In the present case, however, different magnetic states of the catalyst have no effect on the reaction. The same deduction can be drawn a 5 to the reversal of the temperature coefficient of the resistivity of the catalyst. On the other hand, the data of Fig. 3 show that in the neighborhood of the ferromagnetic transition an anomaly is present in the rate of the catalytic reaction. This is in agreement with previous findings of Forestier and Lillc,' who reported anomalies in the rate of the reaction COz H2 S CO H2 on FerOd, SrFez04,NiFeSO, around the Curie temperature of the catalyst. The anomalous behavior of the activity of a catalytic reaction reported in the present communication is quite similar to that previously found for ferroelectric transitions occurring in sodium and potassiuiii niobate and lanthanum ferrite. It is interesting to note that the anomaly sets in somewhat below the Curie temperature. This could be explained by assuming that the ferromagnetic transition occurs on the surface a t a lower temperature than in the bulk.
cis and trans forms of decalin to be 2.4 kcal., which, after correction for heats of vaporization, is in excellent agreement with the experimental observation. '1 siniple inethod of carrying out the calculation of energy difference is as follows trans-Decalin is arbitrarily assigned the value zero; now cis-decalin differs from the trans isomer only in the three skew interactions involving the two polar (1)) boiids as s h o ~ min Fig. 1. The energy difference, hence, is 3 X 0.8 = 2.4 kcal. Applied to the perhydro-phenanthrenes and -anthracenes, this method of estimating energy differences leads to a more definitive picture of the relationships that were established qualitatively earlier, and provitles a criterion for establishing a more precise order of stability. I n addition, this treatment affords an interrelation of the members of the phenanthrene and of the anthracene series on the same energy basis. It should be emphasized, however, that all of these energy differences are for the vapor phase a t 25', and small corrections would be expected2 for the differences in heats of vaporization of the isomers. ( 5 ) H. Forestier and R. Lille, Compl. rend , 204, 265 ( l W 7 \ The perhydroanthracenes (see Fig. 2) are con(6) G. Parravano, J . Chcm. Phys , 20, 3.12 ( l q $ 2 1 veniently considered first. Since the frans-synFRICK CHEMICAL LABORATORY PRINCETOS, YEWJERSEY trum is the isomer of lowest energy, it is arbitrarily assigned the value zero. The cis-syn-trans form then differs from the trans-syn-trans by 3 ske.rcl Energy Relationships of Fused Ring Systems interactions or 2.4 kcal. just as in the decalin series considered above. I n the cis-unti-cis isomer an BY WILLIAMS. JOHXSON additional cis-decalin fusion is imposed which simRECEIVED AUGUST13, 1952 ply doubles the energy difference, i.e., 6 skew interUsing the Pitzer parameters' of 3.6 kcal. for the actions = 4.h kcal. eclipsed, and 0.S kcal. for the skew conformation The tram-anti-trans isomer differs from the (the staggered arrangement being set a t zero) of the fru?i\--cw-frm\ in that the central ring assumes the boat c~nformation,~which accounts for a difference in energy of 5 6 kcal., namely, the difference between the chair and boat form of cyclohexane. I n addition the trans-anti-trans form has two interactions (magnitude uncertain) of the type discussed by Turnerj4which is defined by a rotation of 60" from the staggered conformation. The total energy difference for this isomer, hence, is a value somewhat greater than 5.6 kcal. I n the cis-syn-cis isomer the situation is complicated by the fact that two of the (p) bonds appear in the cis unfavorable 1,3-relationship r=2,4 on the central cyclohexane ring. This interaction is comparable to that of the (p) (p) form of cis-1,3-dimethyl0.8 cyclohexane for which Pitzer' has arbitrarily assigned the Fig. 1. enerm difference of 5.4 kcal. n-butane portion of a hydrocarbon chain, Turnera In the present case the inte&tion of two methylene has calculated the difference in energy between the groups is estimated to be about 8/9 (skew methylene/rnethyl) of this value or 4 8 kcfil i ~ i t 1 1this ( [ ) (dl R 5 p,tTer, cifPmR ~ 27,Rq ~ (1940), ~ , (b) w &.kc{,,
+
'
+
Decafins
1
K
%
P1trt.r and H Syitzer, THISJ O U R N A I , 69. 2.188 ( 1 ~ 4 7 1 T i 'I urner, r b r d , 74, 2119 (1952).
0K
( 3 ) k\' \ I ~ h n ~ o m C x, p e r J r n t l < I , 8 , ? I 7 (1951) ( 4 ) h e r lorrl,,ll.a I S , ref 2 .
