NOTES
The densities of IF7and IOFs as a function of temperature obey the relations IF7: d = (2.7918 f 0.0018) - (0.0049 f 0.OOOl)t
(2.6529 f 0.0003) - (0.00599 f 0.00OO2)t The density of IF7 agrees with the less precise result obtained by Ruff and Keim.l0 The molar refractions of IF7 and IOFs calculated from the Lorenz-Lorentz equation are IOF5: d
IF7: RD = 17.46 cc/mole IOF5: RD = 17.36 cc/mole The dielectric constant of IF7 is 1.75 i 0.08 a t 25' and that of IOF5is 1.97 f 0.08 a t 23". From the foregoing results it can be seen that for IF7 the Maxwell relationship is obeyed within experimental error (nz = 1.66). Presumably IF7 does not deviate appreciably from D5h symmetry as all other possible structures, except for the unlikely D7h, would probably be much more polar. The dielectric constant of IF7 is significantly lower than those of the other halogen fluorides. The specific conductance of IF7 is also extremely low. We have obtained an upper limit of ohm-' cm-l for the pure liquid at room temperature. Cesium fluoride does not appear to be soluble in liquid IF7 and the conductance of liquid IF7 saturated with CsF is not increased measurably over that of the pure liquid. Although certain Lewis acids such as SbF5 and AsF5 are said to form complexes with no complexes of alkali fluorides with IF7 are known a t present.12 We have attempted to react IF7 with CsF a t elevated temperatures with inconclusive results. The foregoing results indicate, therefore, that iodine heptafluoride shows little tendency to behave as an ionizing solvent in contrast to other halogen fluorides. (10) 0. Ruff and R . Keim, Z . Anorg. Allgem. Chem., 193, 176 (1930). (11) F. See1 and 0. Detmer, ibid., 301, 113 (1959). (12) a'. C. Schumb and M. A. Lynch, Jr., Ind. Eng. Chem., 42, 1383 (1950).
Mechanism of Oxygen Reduction Related to Electronic Structure of Gold-Palladium Alloy by A. Damjanovic, V. Brusib, and J. O'M. Bockris The Electrochemistry Laboratory, The University of Pennsylsania, Philadelphia, Pennsylvania 19104 (Received January 24, 1 9 6 7 )
Of Pd-Au In Oxygen reduction at in acid solution it has been observed for the first time
2741
that the change of the dV/d log i slope for the reduction occurs a t a certain alloy composition which can be related to the filling of the d vacancies in the alloys. Since the coefficient dV/d log i often changes with change of reaction mechanism, it follows that the latter depends on the electronic structure of the alloy. This observation is briefly discussed here. The mechanism of oxygen reduction a t a palladium electrode in acid solution is characterized by the first charge-transfer step as the rate-controlling one under Temkin conditions of adsorption of reaction intermediates.' At gold electrodes, with no significant coverage by oxygen-containing species in the potential range in which oxygen is r e d ~ c e d , ~the J first chargetransfer step is also the rate-controlling one but under Langmuirian conditions of adsorption. The kinetics of oxygen reduction in these two cases are very different. At Pd, the Tafel slope is -RT/F and at Au it is -2. RTIF. I n addition to these differences, the activity at, for example, 0.8 v vs. the nhe is lo3 higher at a P d than a t an Au electrode. Au and Pd form a complete series of solid solutions. At four alloy electrodes of different compositions, the rate of oxygen reduction i was determined a t various electrode potentials V. The electrodes were in the form of wires. In Figure 1, Tafel slopes, dV/d log i, are plotted against the atomic composition of the alloys. All Pd-rich alloys have slopes close to -RT/F, as a Pd electrode has. A gold-rich alloy has a slope -2RT/F, the same as an Au electrode. The change of slope occurs a t alloy compositions with more than 25 and less than 50 at. % of Au. Thus, the change occurs a t the point where all the d vacancies are filled. If the electronic structure in Pd-Au changes as in CuNi alloys, it is then expected that a t about 60 at. % or more of Au in Pd-Au alloys, there are no more d vacancies in the d band. An explanation for the change of dV/d log i from -RT/F to -2RT/F would then go along the following line. If for chemisorption of oxygen or of oxygencontaining species a t these two noble metals, d vacancies (or partially filled atomic d orbitals in valence bond treatment) are required, then there is an intermediate coverage by oxygen-containing species a t all alloys with 40 at. % or more of Pd. The mechanism for oxygen reduction a t the Pd-rich alloys corresponds to strong bonding and reasonably high coverage of 0 in's4
(1) A. Damjanovic and V. Brusic, submitted for publication. (2) RI. L. B. Rao, A. Damjanovic, and J. 0'112. Bockris. J . Phys. Chem., 6 7 , 2508 (1963). (3) F. G . Will and C. A. Knorr, Z . Elektrochem., 64, 270 (1960). (4) M. A. Genshaw, A. Damjanovio, and J. O'M. Bockris, submitted for publication.
Volume '71,Number 8 July 1967
NOTES
2742
I
0
x)
Au
1
I00 %
w
by a number of workers, with variations of up to 10% in reported dipole moment.2-6 Similar discrepancies can be noted for the methyl alkanoates, where the series has been measured up to methyl p e n t a n ~ a t e . ~ - ’The ~ current work was undertaken with the intent of resolving these discrepancies and providing sufficient data on these two homologous series to establish firmly the molar polarization increments due to the electronic and vibrational contributions of the -CHz-, -NOz, -CHI, and -COO- groups.
Figure 1. Plot of dV/d log i against atomic composition of Au-Pd alloys.
Experimental Section Materials. The nitroalkanes were purchased from K and K Laboratories and the methyl alkanoates were
termediate, and thus to Temkin conditions of adsorption, and for a first charge-transfer step dV/d log i is -RT/F. At 60 at. % or more of Au, adsorption of oxygen-containing species is low, as found for gold.2J,6 The mechanism for oxygen reduction proceeds then under the Langmuir condition of adsorption and under these conditions, but for the same rate-determining step, dV/d log i is -2RTIF. Though it is known that the kinetics of a number of gas-phase reactions a t a series of alloys show changes a t the concentration where the d band becomes filled,E it appears that this is the first account of a clear change of kinetics of an electrochemical reaction in the region of the critical alloy composition at which d vacancies disappear.’ This work will soon be reported in full. Acknowledgments. The financial support of this work by the U. S. Army Electronics Materials Laboratory, Fort Monmouth, N. J. (Contract No. DA-36039-Sc88921), is appreciated.
provided through the courtesy of Proctor and Gamble. All compounds, including the solvent hexane (Baker Analyzed reagent) , were further purified by distillation on a 28-theoretical plate spinning band distillation column. The purity was then checked on a vapor-phase chromatograph. The chromatographs showed less than 0.5% impurity for every compound, the peaks being spread so as to indicate that the impurities were homologs of the main component. This being the case, the impurities would have negligible effect on the final results. The measurements were made within 1 week of the purification for any given compound and solvent. Equipment. An equal arm transformer capacitance bridge was constructed from a General Radio transformer (Type 941-A) balancing a 400-pf fixed capacitor (General Radio Type 505B) against the sample arm. The sample arm consisted of the sample cell in parallel with a factory calibrated variable precision capacitor (General Radio Type 1422CB) of maximum capacitance 1100 pf which was itself in series with a 500-pf fixed capacitor (General Radio Type 505B). The bridge was grounded a t the transformer center tap. The detector consisted of an amplifier and an oscillo-
(5) B. M. W. Trapnell, Proc. Roy. soc. (London), A218, 566 (1953). (6) G. C. Bond, “Catalysis by Metals,” Academic Press Inc., New York, N. Y . , 1962. (7) Although only a limited number of alloys were studied, the changeover of the mechanism is clearly defined.
The Electric Dipole Moments of Higher Members of the Nitroalkane
and Methyl Alkanoate Series by Ralph D. Nelson, Jr., Charles A. Billings,’ and Michael W. MacIntyre‘ Chemistry Department, Middlebury College, Middlebury, Vermont 06763 (Received J a n u a r y 96,2 967)
The electric dipole moments of the straight-chain nitroalkanes up to nitrobutane have been measured T h e Journal of Physical Chemistry
(1) This work was carried out in partial fulfillment of the requirements for the degree of Bachelor of Arts for C. A. B. and M. W. M. (2) (a) L. G. Groves and 9. Sugden, J . Chem. soc., 158 (1937); (b) E. C. Hurdis and C. P. Smyth, J. Am. Chem. soc., 64, 2829 (1942). (3) C. P. Smyth, ibid., 63, 57 (1941). (4) C. P. Smyth and K. B. McAlpine, ibid., 56, 1697 (1934). (5) R. H. Wiswall, Jr., and C. P. Smyth, J . Chem. Phys., 9, 356 (1941). (6) E. Tannenbaum, R. J. Myers, and W . D. Gwinn, ibid., 25, 42 (1956). (7) K. L. Wolf and W. J. Gross, 2. Physik. Chern. (Leipaig), B14, 305 (1931). (8) R. F. Curl, Jr., J. Chem. Phys., 30, 1529 (1959). (9) S. Misushima and M. Kubo, Bull. Chem. SOC. J a p a n , 13, 174 (1938). (10) C. T. Zahn, Phy8. Rev., 27, 730 (1932).
