3686
on the center atom will stop increasing and start decreasing. It may be that the Br3- ion in PBr7 is past this point. This explains the low value for the charge of the center atom (0.047).
Mass Spectrometric-Knudsen Cell Study
of the Gaseous Oxides of Platinum'
by J. H. Kormaii, H . Gene Staley, and Wayne E. Bell General Dynamics, General Atomic Diuision, John J a y Hopkins Laboratory for Pure and Applied Science, S a n Diego, California (Received February 27, 1967)
The existence of Pt,Oz(g) as a major vaporizing species of platinum in oxygen at -1500" has been demonstrated by Schafer and HeitlandZa using data obtained by Holborn, et aE.,lb who employed a hot-wire method, and by Schneider and E ~ c hElliottY4 ,~ Schafer and Tebben,5 and Alcock and Hooper,s who used transpiration methods. Alcock and Hooper varied the platinum activity by dissolving the metal in gold and showed that x = 1. Also, the kinetics of the oxidation of platinum has been ~ t u d i e d . ~Several '~ of the previous i n ~ e s t i g a t i o n s,6~ . ~ yielded thermodynamic data for the formation of PtO,(g). The present investigation, which used a Knudsen cell-mass spectrometer technique: (1) confirmed the species PtO,(g) ; (2) established the existence of PtO(g); ( 3 ) yielded thermodynamic data for the formation of the two oxide species; and (4) yielded enthalpy data for the Vaporization of platinum metal.
Experimental Section The apparatus and methods used in this study quite closely parallel those previously e m p l ~ y e d . ~An alumina Knudsen cell was used that was loaded with platinum powder (Johnson Matthey, 99.99% purity), placed inside a molybdenum outer crucible, and heated by electron bombardment. The cell was provided with an oxygen-feed tube. The ratio of metal evaporating area to orifice area (orifice diameter, 0.16 cm; length, 0.12 cm) was greater than 100; this seems sufficient for the attainment of equilibrium, on the basis of the results of Fryburg and Petrus,s which indicate that the formation of PtOz(g) is quite rapid under our experimental conditions. Cell temperatures were determined optically by sighting onto the alumina crucible through two 0.16-cm holes in the outer molybdenum cell wall which mas 0.08 cm thick. Furnace conditions The Journal of Physical Chemistry
NOTES
were adjusted to equate the optical temperatures for the top and bottom portions of the Knudsen cell. Appearance Potentials. The ions PtO,+, PtO+, and Pt + were observed and their appearance potentials were measured to be 11.2, 10.1, and 9.0 v, respectively, using the appearance potential of mercury (10.4 v) as a standard. A secondary appearance potential for PtO+ (from PtOz) occurred at 14.5 v. The accuracy of these potentials is believed to be h 0 . 3 v. The measured appearance potential of Ptf is in good agreement with the first ionization potential for platinum (9.0 ~tO.l), according to Iiiser's compilation.1° Identijication of Vapor Species. To identify the vapor species, the oxygen pressure dependence of the intensities of the primary ions was determined a t a potential of 14 v (which is below any determined secondary appearance potentials). Within experimental error, the PtO,+ signal was proportional to the oxygen pressure, PO,, the PtO+ signal was proportional to Po21'2,and the Pt+ signal was independent of PO,,indicating that all three ions were parent ions. These results show that the important vaporization reactions were Pt(s)
+ Pt(s) +
Pt(s)
=
l/*02 0 2
Pt(g) =
PtO(g)
= PtOz(g)
(1) (2)
(3)
under the conditions of our experiments. Vaporization of Platinum. I n measuring the partial pressures of PtO(g) and PtOs(g), it was desirable to use the vapor pressure of platinum as a reference; therefore, a cursory study of the evaporation of platinum was performed. The intensity of the Pt+ signal, I p t , was measured as a function of temperature in the range 1750-2050°K in three separate determinations (about seven points per determination were obtained) without oxygen flowing into the cell. A few points were taken (1) This research was supported in part by the U.S. Atomic Energy Commission under Contract AT(04-3)-164. (2) (a) H. Schafer and H. J. Heitland, Z . Anorg. Allgem. Chem., 304, 249 (1960); (b) L. Holborn, F. Henning, and L. Austin, Abhandl. Deut. Akad. V'iss. Berlin, KZ. Math. Physik Tech., 4, 85 (1904). (3) V. A. Schneider and U. Esch, 2. Elektrochem., 49, 55 (1943). (4) G. R . B. Elliott, UCRL-1831, 1952. (5) H. Schafer and 4.Tebben, Z . d n o r g . A l l g e m . Chem., 304, 317 (1960). (6) C. B. AIcock and G. W. Hooper, Proc. Roy. Sac. (London), A254, 551 (1960). (7) E. K. Rideal and 0. H. Wansbrough-Jones, ibid., A123, 202 (1929). (8) G. C. Fryburg and H. hl. Petrus, J . Electrochem. Sac., 108, 496 (1961). (9) J. H. Norman, H. G. Staley, and W.E. Bell, J . Phys. C'hem., 69, 1373 (1965). (10) R. W. Kiser, TID-6142, 1960.
NOTES
3687
Table I : Pressure Calibration Data
Species
0 0 2
Pt PtO
PtOI a
Mass
Alumina cell, 2018'K-I, mv
- Aa
E
8.0
16 34 195 21 1 227
E - Aa
P, atm
1.78 x 5.5 x (1.25 x 5.5 x 4.7 x
630 123 2.80 0.105 0.073
9.1 5.6 4.5 3.4
10" 10-4 10-7) 10-8 10-0
8.4 9.5
Platinum cell, 1933OK I, mv PPtOz/POz
165 25 8.6
10.8
x
0.045
E, ionizing potential; A , appearance potential.
Table 11: Experimental PtOZ+vs.
T Data A. Platinum Heat of Vaporization Study T,
T,
Ipt+,
OK
mv
OK
mv
OK
mv
2003 1938 2038 1913 1878 1983 1791 1834 1760
9.45 3.10 18.3 2.22 1.10
1943 2043 1851 1750 2003 1908 1802
3.95 21.0 0.885 0.115 11.7 2.25 0.395
1943 2023 1983 2058 1867 1908
2.43 8.40 4.75 15.2" 0.705 1.40
T,
IPt+*
8.05 0.25 0.60 0.105
Ipt+,
B. Platinum &Tonoxide Heat of Vaporization Study T,
IPtOf,
T,
OK
mv
OK
1963 2003 2028 1988 1903 1938 1878
0.085 0.140 0.192 0.115 0.037 0.063 0.030
2003 2048 1978 2028 1933 1918 1958
C.
a
IPtOf,
T,
mv
OK
mv
OK
mv
0.095 0.157" 0.070 0.120 0.047 0.025 0.050
2018 1958 1998 2033 1908 1933 1833
0.152 0.072 0.113 0.170 0.035 0.050 0.025
1973 2008 1928 1993 2038 1913 1958 1898 1873
0.125 0.178 0.073 0.157 0.255 0.065 0.100
IPtOf,
T,
Ipto+,
0,050 0.033
Platinum Dioxide Heat of Vaporization Study
T,
IPtO*+,
T,
IPtOic,
OK
mv
OK
mv
OK
mv
OK
mv
1968 1998 2033 1933 1862 1893 1829
0.194 0.202 0.235 0.155 0.120 0.130 0.090
1893 1993 2033 1840 1933 1802 1760
0.140 0.230 0.265 0.120 0.170 0.095
1978 1835 2013 1683 1883 1745
0.245 0.125 0.265 0.050 0.140 0.070
1958 1824 1689 1883 1739 1978
0.170 0.080 0.035 0.10g
T,
Ipto2*,
T,
IPt02f,
0,050 0.180
0,080
Above Pt melting point.
at temperatures just above the melting point of platinum. I'ieither a vapor pressure nor a heat of vaporization discontinuity was noted; however, the anticipated
heat of vaporization or discontinuity was not expected to be resolvable by the data. The resulting temperature dependence data, plotted Volume 71, Number 11
October 1967
NOTES
3688
as IptT us. 1/T, yielded second-law AHlgoo= 129.5 f 2.5 kcal/mole for reaction 1, where 1900OK is the mean temperature of the measurements. The uncertainties are standard deviations of a single measurement. Combining the AHlgo0value with (HT- H 2 g 8 ) data from Stull and Sinke” gives AH298 = 131.9 f 2.5 kcal/mole. This value agrees within experimental error with the second-law AH298 value of 135 f 2 kcal/mole reported by Dreger and MargraveI2 and with the thirdlam AHzs8values of 135 f 0.85 kcal/mole reported by Dreger and Margrave,l2 135 f 1.0 kcal/mole reported by Hampson and Walker13 and 134.6 kcal/mole calculated by Hampson and WalkerI3 from the data of Jones, et a1.I4 The agreement among the various values is adequate justification for using reported vapor pressures of platinum to calibrate the Knudsen cell-mass spectrometer system for determination of the partial pressure of PtO(g) and PtO*(g). Gaseous Oxide Studies. To determine partial pressure values for the gaseous oxides, P,, intensities, I,+, of P t 0 2 + ,PtO+, Pt+, 02+, and O+ were measured simultaneously at 2018°K I n obtaining partial pressure values from these data, a vapor pressure value of 1.25 X lo-’ atm for platinum at 2018”K, as calculated from the equation given by Hampson and Walker,I3 was accepted as correct. The ratio of cross sections, u ~of, 0 and Pt mas estimated to be 0.107, from information given by Otvos and Stevenson.’j Additivity of atomic cross sections was assumed. 3lultiplier gains inversely proportional to the square root of the molecwere used in the pressure calibration ular weight, dpz, method described by Inghram and DrowartI6 and by eq KIz+T dE, (4) o,(& - A,) 4 where E , is the ionizing voltage, A , is the appearance potential, T is the temperature in OK, and K is the general proportionality constant, This procedure gave partial pressure values described in Table I. The Po/Po1’/’ ratio (7.6 X agrees with the value of 7.7 X 10-4 derived from data of Stull and Sinke.11 This agreement suggests that the method of pressure calibration was satisfactory. The temperature dependence of the intensity of PtO+ was measured in the range 1890-2040°K and that of Pt02+ was measured in the range 1700-2040°K. The oxygen feed to the cell was held at a constant high level during the measurements. Four signal temperature dependence determinations were made for each ion and the data are presented in Table 11. Plots of us. 1/T yielded log (IptoT3’4)us. 1 / T and log (IP~o~T”’) AHZoo0= 100.5 f 4.6 kcalimole for reaction 2 and AHlgoo = 36.6 f 2.3 kcal/mole for reaction 3. The
P,
=
The Journal of Physical Chemistry
fractional exponents associated with T arise because under constant 0 2 flux according to the Knudsen equation the oxygen pressure varied with TI”. Using these values and the measured partial pressures, one calculates A S O Z O O O = 19.5 f 2.3 eu for reaction 2 and As0200, = -5.0 f 1.1 eu for reaction 3, assuming no errors in the pressure measurements (note that the calculation of the entropy for reaction 3 is independent of the pressure calibration and depends only on cross section ratios, gain ratios, etc.). On the basis of the average of heat capacity data for ?”IO(g)-type molecules given by Eielle~,’~CP for PtO(g) is estimated to be 8.45 0.03 X 10-3T 0.83 X 105T-2 cal/”K mole. Combining this result with heat capacity data for Pt(s) and 02,one obtains ACp = -0.94 - 1.46 X 10-3T - 0.69 X 1 0 5 P 2cal/ OK-mole for reaction 2. Combining this value of ACP with the AHnooovalues given above, one obtains AH0298 = 105 f 5 kcal/mole and As02g8 = 24.2 ==! 3 eu for reaction 2. The uncertainties mere estimated. Within the uncertainty, the ASoZg8value is in accord with the typical entropy of formation (20.6 eu) for RIO(g)-type molecules cited (F” - H o o ) / T by Searcy.’* Also, using Bre~ver’s’~ value for PtO(g) and other values from Stull and Sinke,” a AH”, third-law value of 106 6 kcal/mole was obtained for comparison. No CP data for PtOz(g) or similar gaseous molecuIes were found; however, the ACP for reaction 3 is probably near zero. Therefore, the AH and A S values cited above for this reaction are applicable a t other temperatures but with a little greater uncertainty. The enthalpy value for reaction 3 is in good agreement with AH”1430 = 40.7 kcal/mole reported by Schafer and Tebben5 and AHolaoo= 39.3 kcal/mole reported by Alcock and Hooper.6 However, our entropy value for
+
*
(11) D. R. Stull and G. C. Sinke in Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. C., 1956, p 153. (12) L. H. Dreger and J. L. Margrave, J . PhyJ. Chem., 64, 1323 (1960). (13) R. F. Hampson and R. F. Walker, J . Res. .VatZ. Bur. Std., 65A, 289 (1961). (14) H. A. Jones, I. Langmuir, and G. 11. Mackay, Phys. Rer., 30, 201 (1927). (15) J. W. Otvos and D. P. Stevenson, J . Am. Chem. Soe., 78, 546 (1956). (16) hi. G. Inghram and J. Drowart in “Proceedings of the International Symposium on High Temperature Technology,” McGrawHill Book Co., Inc., Piew York, N. Y., 1959, pp 219-240. (17) K. K. Kelley, U. S. Bureau of Mines Bulletin 584, U. S. Government Printing Office, Washington, D. C., 1960. (18) A. W. Searcy in ”Survey of Progress in Chemistry,” Vol. I, A. F. Scott, Ed., Academic Press, Inc., Piew ’I’ork, S . T.,1963, pp 35-79. (19) L. Brewer and AI. S. Chandrasekharajah, “Free Energy Functions for Gaseous Xonoxides,” UCRL-8713 (1959).
NOTES
3689
reaction 3 is appreciably less than the ASo1400= 1.74 eu and As1300 = 0.93 eu reported by these respective investigators. This difference means that our K P values for reaction 3 are about a factor of 10 lower than values found by previous investigators using the transpiration method. Because of the discord in the vapor pressure data for reaction 3, an experiment was performed to determine whether equilibrium pressures were being obtained under our conditions. An all-platinum Ihudsen cell with added platinum wire was employed. The effective orifice area of this cell was about that of the alumina cell used in the previous measurements. The ratio of platinum (evaporation) surface area to orifice area using this system was at least a factor of 40 higher than using the alumina cell. Vse of the platinum cell at 1933°K yielded a pressure ratio Ppto2/Po2that was essentially identical with the ratio found using the alumina cell at this temperature (see Table I), indicating that equilibrium was attained in both measuring systems. The partial pressure data for PtOz determined mass spectrometrically are not in agreement with data obtained by previous investigators using the transpiration method. There appears to be little reason to question strongly the transpiration data. It is possible that the ionization efficiency for PtO, used in our studies is too high; i.e., either electrons are not as effective as suggested by the model of Otvos and Stevenson15or formaICE., tion of undetectable fragments such as Pt+ PtO+ ICE., O+ or 02+is pronounced. Lack of agreement with vapor pressures measured by other methods for certain noble metal oxides has been suspected in other caseszoand calls for a study of ionization cross sections, if at all feasible.
+
+
~~
(20) J. H. Norman, H. G. Staley, and W. E. Bell, "hfass Spectrometric Study of the Noble Metal Oxides: Ruthenium-Oxygen System,'' General Atomic Report GA-7468 (1966).
Ethane Diffusion in Ion-Exchanged Synthetic Zeolites of Type A Containing Water
by W. Rudloff'" and W. W. Brandtlb Departmelit of Chemistry, Illinois Institute of Ttchnology, Chicago, Illinois 60616 (Receiued March 7 , 1967)
The diffusion of gas molecules in zeolites and the self-diffusion of mobile cations in these materials are undoubtedly and strongly dependent on the crystal
structure per se, and on the number and types of structural defects presente2 If one wishes to study the diffusion mechanism, it is at times very advisable to use zeolite samples of identical origin and geometry, modified by ion exchange or by the introduction of "guest" molecules other than the diffusate. In this way, the structural characteristics mentioned above become relatively unimportant and the attractive or repulsive interactions of diffusate, guest molecules, mobile cations, and the crystal lattice, as well as the possible cooperation between these species, can be seen more clearly. This approach has been used by several other aut h o r ~and ~ , in ~ the present study. I t seems to be sound as long as the zeolite structure is not appreciably altered owing to the ion exchange or uptake of guest molecu1es.j The various alkali ion exchanged forms of a zeolite sorbent are of particular interest because model calculations on the ion-molecule and ion-lattice interactions to be expected in some such systems have been reported in the l i t e r a t ~ r e . ~ 'Similarly, ~,~ the idea of cooperation between diffusing and (relatively) stationary species is not new3but calls for careful testing by experiment.
Experimental Section The samples used in the present study have been described earlier.4 The mobile cations were exchanged using the percolation method of Freeman and Stamires.' The mole ratios of [alkali ions introduced]/ [residual calcium (plus sodium) present in the original material] were 1.3, 2.4, and 2.0, for the Li+, Ya+, and I
