NOTES
3326 carbon.14 The study of hydrocarbon-active nitrogen systems thus provides a kinetic tool by which hydrocarbon-H atom reactions can be studied with concentrations of H atoms intermediate between the low concentrations of photolysis experiments and the high concentrations of discharge-tube experiments. Our conclusions concerning cracking of hydrocarbons by H atoms, as studied in hydrocarbon-active nitrogen systems, will be presented in another paper. One other point requires presentation. I n addition t o forming CzH2, reaction 5 also produces CHI radicals. Since C2H2,once produced, moreover, remains an inert constituent of the reaction mixture, the fast reactions that occur subsequent to reaction 5 must be those of CH3radicals with N or H atoms and not those of C2H2. The complexity of these subsequent reactions of CHa radicals is shown by the increased formation of HCN upon addition of H atoms to the CH3C2H N and CH2=C=CH2 N systems. These reactions, however, also require detailed presentation in another paper and are, again, not discussed further here. The conclusions reached in this investigation also suggest that the addition of CHaC2H to a stream of H atoms might have use as a source of CH3 radicals for kinetic studies because of the inertness of CzHz to subsequent consumption by reactive species. We also plan t o investigate this possibility further. The reaction of CH3C2H with H atoms, however, probably could not be used to obtain accurate estimates of H atom concentrations (by measuring the CzHzproduced), since H atoms will be rapidly combined by the C2H2 produced by reaction 5, owing to the occurrence of the fast reactions 11 and 12, kover-sl~= 10-14 cm3/molecule ~ e c , l 4 in , ~ ~competition with reaction 5 .
+
+
modynamic calculations, thermogravimetric analysis, and X-ray diffraction measurements of the present study have indicated that it is an oversimplification to assign these activation energy values to the formation of SrTi03. In solid-solid reactions, according to Garner,s a product layer is formed a t the contact boundary between reactants. The reactions proceed by diffusion of the reactants through the product layer and by phase-boundary processes which cause consumption of the original crystal lattices. Concentration gradients in the product layer permit the formation of compounds with stoichiometries other than that of the original reactant mixture. The solid-solid reactions of BaC03 TiOz were studied by Templeton and Pask.6 They reported that BaTi03 was formed by an initial contact reaction, and subsequent diff usion-controlled reactions formed all possible compounds indicated for a given temperature in the phase diagram of Rase and ROY.’ The heats of reaction and equilibrium C02 pressures for some possible reactions of SrC03 Ti02 were calculated from the data of Coughlin,*Lander,s and Kelley, Todd, and King.10 The calculated values are summarized in Table I for the temperatures 1000, 1200, and 1400°K. The calculated equilibrium COz pressures for reactions 1-4 are shown in Figure 1 for the temperature range of 800-1400°K.
+
+
7
Q7
(16) J. V. Michael and H. Nilri, J . Chem. Phys., 46, 4969 (1967).
Solid-state Reactions of SrC03
+ Ti02
by Tu’. H. Harris and R. L. Cook University of Illinois, Ceramic Engineering Department, Urbana, Illinois 61806 (Receiued October 6 , 1967)
The reaction kinetics of the formation of SrTi03 from SrC03 Ti02 (rutile) were studied by Hanykyrl in the temperature range of 812-1356’. Activation energies were reported for the formation of SrTi03 to be 55.4 kcal/mol by reaction of 0.26 p (average particle size) of SrCO3 with 0.19 p of Ti02 and 55.6 kcal/mol with 50-60 p of TiOz. Kinetic data were obtained from chemical determination of unreacted SrC03. Activation energy calculations were based on kinetic models of Jander,2 Carter,3 and Dunwald and Wagner.4 Ther-
+
The Journal of Phylaieal Chemistry
// -LDry
I * ’ 800
Air C o p Pressure 1200
1000
Temperature
1400
O K
Figure 1. Calculated equilibrium COZpressures us. temperature (OK) for reactions 1-4 of SrCOa TiOz(1:1 mole ratio).
+
(1) V. Hanykyr, Silikaty, 10, 17 (1966). (2) W.Jander, 2. Anorg. Allgern. Chem., 163, 1 (1927). (3) R. E. Carter, J. Chem. Phys., 34, 2010 (1961); 35, 1137 (1961). (4) H.Dflnwald and C. Wagner, 2. Phvsilc. Chem. (Leipzig), B24, 53 (1934). (5) W.E.Garner, “Chemistry of the Solid State,” Butterworth and Co. Ltd., London, 1955,p 297. (6) L. K.Templeton and J. A. Pask, J . Amer. Ceram. SOC.,42, 212 (1959). (7) D. E.Rase and R. Roy, {bid., 38, 102 (1955). (8) J. P. Coughlin, U. S. Bureau of Mines Bulletin 542,U. 8. Government Printing Office,Washington, D. C., 1954. (9) J. J. Lander, J. Amer. Chem. Soc., 73, 5794 (1951). (10) K. K. Kelley, S. S. Todd, and E. G. King, Report of Investigations 6059,Bureau of Mines, Pittsburgh, Pa., Oct 1954.
KOTES
3327 Calculated Heats of Reaction and Equilibrium COZPressures for Reactions of SrCOa -b Ti02
coz Temp, Reaction
SrCOa(s) + SrO(s)
SrCOs(s:i
+ TiOz(s)
2SrCOs(rr)
+ COz(g) -P
+ TiOZ(s)
SrTiOds)
+
+ COz(g)
SrzTiOds)
a
+ SrTiOa(s)
-+
SraTiOds)
AF is the free energy of the reaction.
,
1000 1200 1400
+ 2coz(g)
1000
+ COz(g)
AF,a kcal/mol
2.98 x 10-4 1.77 x 2.00 x 10-1
1000 1200 1400
1200 1400 SrCOs(s)
premure, atm
OK
3 800 17,340 24,600 2.72 38.9 126 1.91 x 108.67 X 10-2 6.31 X lo-'
1000 1200 1400
AH,* kcal/mol
16.4 9.43 4.48
51.5 46.1 44.6
-16.4 -23.3 -28.1
21.4 15.7 15.8
-3.96 -17.4 -26.8
60.7 48.6 47.1
12.4 5.83 1.28
48.5 42.9 41.3
A H is the heat of the reaction.
The calculated GOz pressures of reactions 2 and 3 are much greater than reaction 1, so that formation of a compound would be an easier path for SrC03 decomposition. However, the formation of SrO as an intermediate step would be thermodynamically possible, as indicated by combination of reactions 1 and 2 and of reactions 1 and 4
+ TiOz -+ SrTi03 SrO + SrTi03 +SrzTiO4
phases of SrC03, TiOz (rutile), and SrTiOa. However, the samples heated 6 hr a t 1100" indicated the presence of SrC03, TiOz (rutile), SrTi03, SrzTi04,Sr3Ti207,and SrrTi3010.
Table I1 : Impurity Content of Raw Materials
Sr0
Impurity,
%
SrCOa (99.67% pure)
According to Figure 1, reaction 1 would not be possible in the temperature range 800-1400°K if the COZpressure were 1atm or greater. However, under a vacuum or in 1 atm of air or inert gas, reaction 1 could not be ruled out as a necessary intermediate or alternate path. Thermodynamic data are not available to permit calculations for reactions forming Sr3Tiz0, and SsTi301a. I n the present study an activation energy of 53.7 kcal/mol was obtained from thermogravimetric analysis of SrC03 TiOz (1:1 mole ratio) in the temperature range of 950-1 100". The isothermal thermogravimetric measurements were conducted in an ambient air atmosphere. Reagent grade SrC03(Mallinckrodt Chemical Works, New York, N. Y., 99.67% pure) and highpurity TiOz (National Lead Co., South Amboy, N. J.,Lot No. MP-2254 9!3.98% pure) were used as raw materials. The raw-material impurity contents are shown in Table 11. The TiOzwasdetermined by X-raydiff ractometry to consist almost entirely of the rutile form. The highest intensity anatasle line (101) was barely detectable, while that for rutile (1.10) was observed to be very intense in the TiOz sample as received. Average particle sizes of 0.6 p for SrC03 and 0.8 p for TiOz were obtained by direct-transmission electron microscope observation. X-Ray diffraction measurements a t 25" for SrC03 TiOz (1 : 1 mole ratio) heated 6 hr a t 950" showed only
+
+
Alkali salts (sulfates)
0.300
B&
0.005 0.005 0.002 0.002 0,005 0.010
c1 Pb Fe
804 Acetic acid (insoluble) Ti02 (99.98% pure) Si02 Fez08 Alzos SbnOa Mg CU Pb Mn
Ni
V Cr
0.003
