2432
WAYNE
E. BELLA S D M. T A G A M I
R=C2Hs in each series, the extent of the shift of electron density away from the or-carbon or the actual enhancement of charge transfer toward the electronegative X which is due to the electropositive inductive effect of the additional methyl group seems t o be more important in a longer than in a shorter bond. Thus, for the RI series, the relative change in electron shielding on the a-carbon per substitution of hydrogen by a methyl group is of greater magnitude than it is in the RF series, the relative changes in all other groups being intermediate. That the relative spatial displacement of the center of gravity of the negative end of the dipole moment or additional increment in charge transfer toward X in R X series with the increase in the inductive effect of R is more significant for longer C-X bonds, in going from CHaX to C2HSX, for example, seems to be borne out by the dipole moment measurements of the methyl and ethyl halides listed in Table 111. TABLE I11 DIPOLEMOMENTS OF METHYL AND ETHYL HALIDES X
PCHsX
PCZESX
PCHaX
- PCaPHsX
Reference
F
1.81 1.92 0.11 a c1 1.83 2.00 .17 b Br 1.75 1.99 .24 b I 1.60 1.93 .33 b C. P. Smyth and K. B. McAlpine, J. Chem. Phys., 2, 499 (1934). P. C. Mahanti, Phil. Mag., (7) 20, 274 (1935).
We see that the increments in dipole moments in going from CH3Xto C2HsXfollow the order of increas-
VOI.
ti7
ing C-X bond lengths, which in this case is opposite to the order of electronegativities. It is difficult to explain the relative constancy of the slopes in &C13 shifts. The small differences in these slopes, however, do exist and may still be significant. Thus in the halogen series (X = GI, Br, I) the slopes still follow the order I > Br > C1, but then the slope for X = COOH is even greater than that for X = I. It seems, therefore, that the induced polarity of the C-X bond may have only a second-order effect on these slopes and that some other factors predominantly account for the rather significant chemical shifts of /&carbon resonance toward lower field per methyl substitution. We do not feel prepared to advance any plausible explanation for these effects. Also, the evidence presented for hyperconjugation effects cannot be regarded as conclusive. On the other hand, if our tentative explanation of the large variation in the slopes of a-carbon plots is correct, the bond lengths between the carbon in question and substituent groups should be considered together with other factors such as electronegativities, types of hybridization, and magnetic anisotropies of various groups in future theoretical appraisals of chemical shifts. Acknowledgments.-The authors wish to thank Professor C. P. Nash for his helpful suggestions and Professor W. E. Thiessen for proofreading the nianuscript.
HIGH-TEMPERATURE CHEMISTRY OF THE RUTHENIUM-OXYGEN SYSTEM' BY WAYNEE. BELLAKD M. TAGAMI General Atomic Division of General Dynamics Corporation, John Jag Hopkins Laboratory f o r Pure and Applied Science, S a n Diego, California Received M a y 28, 1963 The ruthenium-oxygen system has been studied over the temperature range 800 to 1500" and over the oxygen pressure range 0.01 t o 1.0 atm. Results show that solid RuOz is the only stable condensed oxide under the conditions of the study. The dissociation pressure of the oxide reaches 1 atm. a t 1540'. The effect of oxygen pressure on vapor pressure indicates that the important vapor species are RuOa and RuOi. From the pressure data, the following heats of formation, AHozss,and standard entropies, X"Z~S,were obtained: -72.2 f 2.0 kcal./mole and 12.5 f 2.0 e.u. for RuOz(s), - 18.0 f 4.0 kcal./mole and 63.7 f 4.0 e.u. for RuOt(g), and -46.7 f 5.0 kcal./mole and 65.5 f 5.0 e.u. for Ru04(g).
Introduction
As part of a systematic investigation of the high-temperature chemistry of transition element compounds, the ruthenium-oxygen syste 1 was studied. Condensed phases and vapor species were identified, dissociation and vapor pressures were measured, and therniodynamic data were calculated from the pressure data. Prior to the start of this work, literature on the behavior of the ruthenium-oxygen system a t high temperature was scant and to some extent discordant. Remy and Kohnza reported a few dissociation pressure data for RuOz(s). Alcock and Hooper2bstudied the volatility of ruthenium in oxygen as a function of temperature in the range 1200 to 1400' and as a function of oxygen pressure a t 1280'. They found the vapor pressure t o be propor-
tional t o p ~ and, ~ assuming ~ / ~the solid phase to be ruthenium metal, deduced the vapor species to be RuzO. Schafer, Gerhardt, and Tebben3studied theeffect of oxygen pressure on vapor pressure a t 800' and in the range 1465 t o 2090' and found evidence for the vapor species Ru04and Ru03. They observed the dissociation pressure of RuOz to be much less than that reported by Remy and Kohn. Recently, Schafer, et a1.,4,5have completed an exhaustive study of the ruthenium-oxygen system at high temperature. We mere pleased to receive preprints of publications covering their work and find good agreement between their results and the results of our study. Experimental Dissociation Pressure Studies.-Dissociation
pressures were
(1) This work w a s suppoited in part by the U. S . Atomic Energ> Coni-
mission under Contract AT(04-31-164. ( 2 ) (a) H. Remy and M. Kohn, Z. anorg. allgem. Chem., 137, 365 (1924); (b) C. B. Alcock and G. W. Hooper, Proc. Roy. Sue. (London), 6 2 5 4 , 551 (1960).
(3) H. Schafer, W. Gerhardt, and A. Tebben. Angew. Chem., 73,27 (1961). (4) H. Schafer, G. Schneidereit, and W. Gerhardt. 2. anorg. allgem. Ckem.. 319, 327 (1963). (5) H. Sohafer, A. Tebben, and W. Gerhardt, abid., 321,41 (1963).
Nov., 1963
HIGH-TEMPERATURE CHEMISTRY OF
measured by both static and dynamic (transpiration) methods using techniques similar to those described in a previous papern6 In the static method, the oxide sample was contained in a deadend mullite reaction tube and dissociation pressures were read on a mercury manometer. A small sulfuric acid manometer was used as a sensitive indicator to show when the pressure in the mercury manometer was equal to the pressure in the reaction tube. For the transpiration method, a mullite reaction tube was used.7 Helium served as the carrier gas and the effluent heliumoxygen mixture was analyzed by use of a gas chromatograph. Flow rates of the carrier gas were in the range (2 to 5 ml. STP/ min.) where measured oxygen pressures were found to be independent of flow rate. Vapor Pressure Studios .-Vapor pressures were determined by the transpiration method using techniques similar to those used in previous ~ t u d i e s . ~Mullite reaction tubes were used, and oxygen normally served as 1,he carrier gas. The effluent oxygen gas was collected over mercury in a known volume a t reduced pressure. A correction was made for the oxygen released on the condensation of the oxidle vapor. Flow-rate studies were conducted a t 802, 1303, and 1453' and the results show that a t the flow rates used (0.01 to 0.30 mmole of Oz/min., depending on temperature and oxygen pressure conditions) equilibrium conditions were established and diffusion effects were not important. To permit working a t pressures of oxygen as low as 0.01 atm. a t temperatures around L500°, argon-oxygen mixtures were tried as carrier gases. However, in each case, it was found that the oxygen content of the mixture decreased appreciably (20 t o 30%) in passing through the reaction tube as a result of the formation of ruthenium oxide (solid and vapor). It was simple to analyze the effluent gas mixture mass spectrometrically , but how the oxygen content of the mixture varied during the course of the experiment was quite difficult to determine. A radiotracer method of analysis was used to determine the quantity of ruthenium transpired. The ruthenium metal used was irradiated in a TRIGA reactor to produce the radionuclide 41-day Ru103. Three different metal samples having activities in the range of 1,000 to 12,000 c.p.m./mg. under our counting conditions have been used. y-Ray spectra of the samples agreed with spectra reported for Rulo3. At the end of each experiment, the mullite condensing re,gionwas crushed, placed in a plastic vial, and counted in a u-ell-type counter. Optimum counting conditions were obtained by (countingthe main y-energy peak of Rules (0.50 Mev.) through a window. To minimize geometry problems, each sample was (counted a t least five times after being shaken between each count. Statistical counting errors were less than 2y0 standard deviation. For standards, samples of the radioactive metal were weighed out, mixed with crushed mullite, and counted in the same manner as the unknowns. Bctivities of the unknowns ranged from about 200 to 25,000 c.p.m. The background was about 20 c.p.m. As a check on analytical procedures, two vapor-pressure measurements were made a t each of the temperatures 902 (903), 1002 (3003), and 1303' using different ruthenium metal samples. Agreement among the values obtained (see column 2 of Table I) was satisfactory. It should be mentioned that the same ruthenium metal sample was used throughout the studies on the effect of oxygen pressure on vapor pressure a t each temperature, General.-The materials used were ruthenium metal sponge (Johnson, Matthey, 99.995% purity), oxygen gas (Matheson, research grade), argon gas (Liquid Carbonic), and helium gas (Liquid Carbonic). The oxygen flowed through a sulfuric acid bubbler and P2Oj before entering the reaction tube. The helium was purified by passage through a charcoal trap held a t liquid nitrogen temperature. A loyo platinum-rhodium wound tube-furnace was used. P t us. Pt-lOyo Rh thermocouples were used. Temperature unmrtainties are believed to range from f 2 " a t 800" to r t 4 ' a t 1500'.
Results and Discussion Condensed Phase Studies.-It was found that hea,ting of ruthenium for several days in oxygen at 950" yields an oxide having an oxygen content within 1% (6) W. E. Bell, (1960).
hJ. C. Garrison, and U. AIerten, J . Phys. Chem., 64, 145
(7) W. E. Bell, U Merten, and AI. Tagami, rbad., 66, 510 (1961).
THE
RUTHENIUI\I-~XYGEN SYSTEM I .o
I
I
2433 I
0.5
0.2
4
0,I
I
-G$
0.05
ln 3
3 a:
a W z 0 >
0.02
X
0
0.0I
0.005
0.002
0.00I
II
I
x 103. pressure of RuOn(s).
IOK
Fig. 1.-Dissociation
of the theoretical value for RuOz. Other investigators have reported the preparation of RuOz(s).2a To test for lower oxides and also to determine oxygen solubility in the metal, a sample of the oxide contained in a dead-end mullite tube was decomposed a t 1450" and a t an oxygen pressure that was a few millimeters below the dissociation pressure at 1450'. The sample then was sealed off, quenched in air, and analyzed gravimetrically. The resulting material was obviously metal and contained less than 0.40 atom % oxygen. In a further test for lower oxides, a mixture of Ru(s) and RuOz(s) was made in which ruthenium and oxygen were in the atomic ratio 1 to 1. The mixture was sealed in a small evacuated quartz tube, annealed a t 1000" for 48 hr., quenched, and then analyzed by X-ray techniques. Diffraction lines were found only for Ru(s) and RuOz(s). From the results of these tests and from the fact that the degree of oxidation had no effect on dissociation pressure data (see below), we conclude that RuOz is the only stable condensed oxide under our experimental conditions. Dissocation Pressures.-Dissociation pressure data measured over the range 11Oi to 1503' by two different methods are plotted in Fig. 1. The data are linear on the log p us 1/T plot within experimental error and extrapolate to 1 atm. oxygen pressure at 1540O. From the slope of the line drawn through the points in Fig. 1, me calculate a t the mean temperature of the = -67.4 =t1.0 kcal./ measurements (1570'K.) mole and from this we obtain AXo1570 = -37.2 f 0.5 e.u. for reaction 1. ( 8 ) F. Krauss and G. Schiader 2. anorg. allgem. Chem., 176, 383 (1928). (9) G. Londe, h d , 1 6 3 , 345 (1927).
WAYNEE. BELLAND M. TAGAMI
2434 50
I
,
I
0.I
I
0.2
0.5
I
1.0
OXYGEN PRESSURE (ATM.).
Fig. 2.-Effect
of oxygen pressure on vapor pressure at 1303, 1453, and 1503'.
Ru(s)
+
0 2
=
RuOz(s)
VOl. 67
They observed a sharp decrease in the dissociation pressure of RuOz on mixing a small quantity of ruthenium metal with the oxide. They attributed this behavior to slight solubility of metal in the oxide. Ortner, Anderson, and Canipbellla made rough measurements of the dissociation pressure of Ru02 in the range 1000 to 1450'. Their results, which were reported in the form of an equation, are in reasonable agreement with our results. They calculate = -75 kcal./mole for reaction 1. Recently, Schafer, Schneidereit, and Gerhardt4measured the dissociation pressure of RuOz using glowingfilament, therinobalance, and static techniques. They estimated 8'298 RuOt(s) = 14.5 e.u. using Latimer's tables14 and calculated the third-law value AH'29s = -71 kcal./niole for reaction 1. Their results indicate that the oxygen pressure reaches 1atm. a t 1580'. Identification of Vapor Species.-Since Ru and RuOz are the stable condensed phases in the temperature range and in the oxygen pressure range studied, the solid-vapor equilibria to be considered ai-
(1)
Using AC, = 3.7 f 1.0 cal./deg.-mole, estimated froin heat capacity values for the elements and for solid metal dioxides given by Kelley, lo we calculate (assuming AC, to be constant) AH'298 = -72.2 f: 2.0 kcal./mole; A8'298 = -43.3 f 2.0 e.u.; and log PO, (atm.) = -16,002/T - 1.862 log T 14.89. The heat of formation agrees with dH0298 = -73 f 1 kcal./mole obtained calorimetrically by Shchukarev and Ryabov.ll Combining Ax02Q8with standard entropies S O 2 9 8 Ru = 6.82 6 0.05 e.u. and x'298 O2 = 49.01 f 0.01 e.u., given by Kelley and yields 8'298 RuOz(s) = 12.5 f 2.0 e.u. This value seems reasonable when compared with standard entropy values given by Kelley and King12 for similar oxides: 12.68 f: 0.10, MnOz; 13.03 f 0.07, NbOz; 12.04 f 0.04, TiOa; 12.3 f 0.2, VOz; 12.12 f 0.08, ZrOz. A slight change in dissociation pressure with oxygen content was observed. For example, in static measurements a t 1380" which were begun with a sample of oxide-coated metal particles (X-ray analysis showed that the particles were almost completely converted to the oxide), the first observed vapor pressure was 0.23 atm. On oxygen removal, the dissociation pressure rapidly decreased to 0.16 atm., a t which point further removal of oxygen had no effect. This behavior can be attributed to adsorbed oxygen (or gaseous impurities) or to a small range of homogeneity in solid RuOz. Except for the initial effect just mentioned, the oxygen content of the sample had no apparent effect on the dissociation pressure values observed a t the temperatures of study. This is another indication that solid oxides with oxygen content lower than RuOz are not thermodynamically stable under our conditions. Remy and Kc hn2 reported dissociation pressure data for RuOz in the narrow temperature range 923 to 932". Their pressure values exceed our values in this temperature range by more than two orders of magnitude.
Froin the equilibrium constant for reaction 2, we obtain
+
(10) K. I
