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RICHARD F. PORTER AND R. C. SCHOONMAKER
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powders studied, the temperature a t which sintering set in corresponds roughly with the temperature a t which surface defects are formed. Recently ParravanoZ6proposed that the sintering mechanism in metal oxides consists of welding crystallites together by "bridges" which emerge from surface defects. Our results tend to support this hypothesis.
about mm. is approximately 875'. Reversible oxygen adsorption, if present, is too small to be detected. (3) Nearly all water vapor and other condensable vapors appear to be desorbed a t 800' in high vacuum. Presumably, an analysis of the composition of low area titanium dioxide subjected to these conditions should yield a compound close to the stoichiometric composition. Conclusions (4) Ndther conclusion (2) nor (3) was altered Several conclusions can be cited from this work. by doping the purified TiOz with various amounts (1) Reversible oxygen adsorption seems to of BaO, NazO or TiF4. However, NazO incorpooccur at surface impurity centers in commercial rated in the lattice is volatile above 875' and ingrade titanium dioxide a t temperatures between corporated TiF4 is volatile between 400 and 800'. NOTEADDEDIN PROOF.-under the conditions of our ex500 and 900'. This finding is in agreement with periment, the NaO undoubtedly vaporized from the doped reports that isotopic oxygen exchange is obtained Ti02 samples as sodium atoms and oxygen molecules." with commercial and/or reagent grade samples of Acknowledgments.-The authors thank the NaNiO, CaO, MnOz, TiOz, Thoz and 7-A12032s-29 a t temperatures as low as one-fourth the melting tional Lead Company and the Electro Metallurgicil Company for contribution of the Ti02 powder point of the bulk phase. and granular titanium metal, respectively. They (2) The temperature of incipient decomposition of highly purified TiOz in a dynamic vacuum of also thank the Research Corporation for its h a m cia1 support of this study and the National Science Foundation for its grant to A. W. Czanderna. (25) G. Parravano, personal communication. (26) J. A. Allen and I. Lauder, Nature, 164, 142 (1949). Finally, thanks are due to Dr. A. F. Clifford and (27) N. Morita, BUZZ. Chem. SOC.Japan, 16, 1 (1940). Mr. Q. W. Choi for consultation and suggestions. (28) E. R. S. Winter, J . Chem. Soc., 1170 (1950). (29)
E. Whalley and E. R. 8. Winter, ibid., 1175 (1850).
(30) Leo Brewer, personal communication.
A MASS SPECTROMETRIC STUDY OF THE VAPORIZATION OF FERROUS BROMIDE1 BY RICHARD F. PORTER AND RICHARD C. SCHOONMAKER Department of Chemistry, Cornell University, Ithaca, New York Rsciivad October I,1.968
A mass spectrometer has been used to analyze the vapors effusing from a Knudsen cell containing FeBr2(s). I n the temperature interval 620-665OK., the monomer is the predominant vapor s ecies; but at the melting point the dimer concentration becomes si nificant. Thermochemical data have been determineifor the reactions: 2FeBrds) = Fe2Bra(g), A H o w = 59.5 & 6 kcal.,fmole dimer; Fe2Br4(g)= 2FeBrz(g), AHOW = 34.7 i 4 kcal./mole dimer.
Introduction Unlike the iron(II1) bromide which is known to vaporize largely as gaseous dimers, the iron(I1) bromide is believed to vaporize primarily 8.8 the monomer, FeBr2(g). Evidence that FeBrz is not appreciably associated as dimers in the vapor phase has been obtained in the vapor pressure studies of MacLaren and Gregory.2 Employing effusion, transpiration and diaphragm methods of measurement, these investigators determined vapor pressures of solid and liquid FeBrz in the temperature range 400-909". Based on the three independent methods their data give reasonably concordant vapor pressures if the monomer is assumed to be the major vapor species. The authors point out, however, that in the higher temperature range near the melting point of the solid, the presence of about 15% dimer cannot be excluded. In order to obtain information on the possible (1) This research was aupported by the U. S. Air Force through the Air Force O5ce of Scientific Research of the Air Reaearch and Development Command under contract No. AF 18 (603)-1. (2) R. 0. MacLaren and N. W. Gregory, THIBJOURNAL,S9, 184 (1965).
existence of polymeric species, we have analyzed, mass spectrometrically, the vapors effusing from a Knudsen cell containing solid, anhydrous ferrous bromide. Earlier mass spectrometric studies on ferrous chloride2indicated that FeClz is associated to some extent as dimers in the saturated vapor. Experimental Descriptions of the experimental method and the furnace assembly for a 12-inch, direction-focusing mass spectrometer have been given earlier.sajb The method involves electron bombardment of the vapor leaving an effusion cell and the subsequent identification and study of the positive ions produced. The experimental conditlons closely approxlmated those previously reported for FeClp. Effusion cells were constructed of high purity iron and the ratio of orifice area to total sample surface area was less than 5 X lo-'. The temperature range investigated was between 300 and 400". Temperatures were measured with a chromel-alumel thermocouple calibrated at the melting point of zinc. Samples of FeBr2 were prepared by passing anhydrous HBr over red hot iron wire in a Vycor tube. Yellow flakes of FeBr2, collected on the cooler portion of the tube, were used. Results from the first few experiments were discarded be(3) (a) R. C. Schoonmaker and R . F. Porter, J . Chem. Phys., 29, 116 (1958); (b) R . F. Porter and C. Sohoonmaker, THIUJOURNAL, 6'2, 234 (1958).
R.
April, 1959
SPECTROMETRIC STUDY OF THE VAPORIZATION OF FERROUS BROMID~
627
cause of interferin hydrolysis of the samples. This was recognized by the %igh HBr+ background and a black residual surface layer on the samples after heating. With sufficient recautions to kee the sam les dry during prepathe fresfly prepared material ration ana rapid transfer to the mass spectrometer, hydrolysis was diminished and satisfactory results could be obtained. At temperatures above 400' ion current ratios were not, in general, very reproducible. This condition probably was caused by failure to attain complete vapor saturation due to surface contamination of the solid resulting from hydrolysis. A large, discontinuous and non-reproducible increase in the ratio IFeBr*/IFeBi# was observed at higher temperatures during several runs. At present, this anomalous behavior remains unexplained.
07
Results and Discussion A typical mass spectrum of ferrous bromide vapor is shown in Table I. Identification of ionic species with masses below 200 was obtained by counting from known background masses (primarily residual hydrocarbon). Ions with mass over 200 were checked with a Rotating Coil Gaussmeter. Final identification of all ions was made by comparison of observed intensities of isotopic species with those calculated from the natural abundances of Fe and Br isotopes. Since all of the isotopic masses are two units apart, no difficulty was encountered in resolving the heaviest isotopic species. Masses containing three or four ahoms of Fe were not detected. Appearance potentials for the principal ions in the vapor, calibrated against the ionization potential of HBr+, are presented in Table I. The ions observed fall TABLEI
MASS SPECTRUM OF FeBrz VAPOR (T
= 392') Appearance otentialb
Ion Mass no. Intensity" C".? Fe 56 45.0 16.6 f 0 . 5 FeBr + 135 63.5 12.9 f . 5 FeBrz + 216 100.0 10.7& .5 191 0.8 . ..... FezBr + FenBrz 272 0.3 . .... . . . FezBri + 351 9.3 13.6& .5 431 5 6 12.6& .5 FezBrr + a Relative units; ionizing electron energy = 50 volts. Obtained by linear extra olation of appeltraiice potential curves, calibrated against W(HBr+) = 13.2 volts. +
+
into two classes: those coli taining one atom of Fe, and those containing two atoms. Below 400°, the ratios IFeBr + / I F e B n + and IFelBra +/IFeeBr3 had only a slight dependence on temperature (see Fig. 1) indicating that FeBr+ and Fe2Br8+ are probably formed primarily from the parent molecules of FeBr2+ and FeBr4+, respectively. The slight temperature dependence of these ratios may be due to a temperature dependent dissociative ionization cross section, or, in the case of I F e B r + / I F & * + , to a small contribution from a second process, e.g., dissociative ionization of the dimer. The presence of stable molecules with iron in an oxidation state lower than two ( i e . , FeBr(g), Fe2Bra(g), etc.) is more or less ruled out since the solid vaporizes stoichiometrically as FeBrz. The intensity ratio of FeBr2+ to FezBr3+ varied considerably with temperature as indicated in Fig. 1. This is taken as sufficient evidence that FeBr2+is not formed primarily from the parent of FezBr3+ or Fe2Br4+. The observations are coiisistcnt with +
I/T(x
Id).
Fig. 1.
the presence of two molecules, FeBr2(g) and FezBrd(g), in the vapor phase. The high appearance potential (-16.6 volts) of the Fe+ ion leads to the conclusion that it is formed by fragmentation. A possible process for the formation of Fe+ is FeBr2(g)
+ e- +Fe + 2Br(g) + 2e+
The mechanisms by which Fe2Br+ and Fe2Br2+ are formed could not be determined due to the low intensities of these species. However, it seems reasonable to attribute their formation to dissociative ionization of FezBrd(g). Since our temperature range overlaps the effusion data of MacLaren and Gregory, we can use their vapor pressures to calibrate the mass spectrometer. For our instrumentlaaP,, = TK,,Ixl+, where P,,is the partial pressure of species xi in the effusion cell, Ixi+is the total intensity of the ion species Xi+ corrected for isotopic abundance, T is the absolute temperature, and Kxi is an instrument sensitivity factor incorporating ionization cross section and ion detection efficiency terms. Thus, for a given temperature
I n this expression a(FezBr4)/a(FeBrz) is the ratio of relative partial ionization cross sections for Fe2Br4(g) and FeBr2(g) ; and X(Fe2Br4)+/S(FeBr2+) is the ratio of relative electron multiplier detection efficiencies for FeBr2+ and FezBrd+. If the ionization cross section ratio, a(Fe2Br4)/a(FeBr2), is taken to be 2 and the multiplier efficiency term to be approximately ldSl4then
Equilibrium pressures of FeBrz(g) and Fe2Br4(g) a t several tempera tures, calculated from our ion intensity ratios and the effusion data of MacLaren and Gregory, are presented in Table 11. Since AFOT =
( P F ~ B - RT In -
~ ~
PFerBrr
for the rcaction Fe2Br4(g) = 3FeBr2(g),a heat of
RICHARD F. PORTER AND R. C.SCHOONMAKER
628
Vol. 63
dissociation may be determined if an entropy is evaluated for the reaction. Experimental reaction entropies have been determined2 for the dimerization of FeClz(g), FeCls(g) and FeBrs(g). The values are not widely divergent, and from them we estimate -28 f 5 e.u. for the entropy of dimerization of FeBrz(g). Using this entropy and values of AFOT, we have calculated the heats of dissociation for Fe2Br4(g) = 2FeBr2(g) presented in Table 11. Combination of the heat of dissociation of FezBrr(g) with a value of 47.1’ kcal./mole for the process FeBra(s) = FeBr2(g) yields the heats of TABLEI1
FezBrr(g) of 59.5 kcal./mole determined in the present work, and assuming that AC, (sublimation) for FeBrz(g) and Fe2Br4(g) are approximately equal to those e ~ t i m a t e for d ~ FeClz(g) ~ ~ ~ ~ and Fe2C14(g), respectively. At the melting point of FeBrz, we obtain a total vapor pressure of 3.0 X atm. which, though perhaps fortuitous, is in good agreement with the value 2.95 X atm. calculated from the vapor pressure equation of MacLaren and Gregory which was obtained from diaphragm gauge measurements where no assumption was necessary concerning the vaporizing species. The data in Table I11 indicate approxi-
THERMOCHEMICAL DATAFOR THE REACTIONS: (a) FezBrr(g) 2FeBrz(g); (b) 2FeBrz(s) = FezBn(g)
TABLE111 MONOMER/DIMER PRESSURE RATIOFOR FERROUB BROMIDE AT VARIOUS TEMPERATURES VAPOR
-
T (OK.)
pFeBr,(atm.)
AFTa AH TO^^ kcal.) koala/ mole mole dimer dimer reacrempFerBr4(atm.) tion (a) tion (a)
2.76 x 10-8 1.08 x 10-O 17.4 34.8 6.05 x 10-8 2.54 x 10-0 17.0 34.8 1.07 X 10-7 4.54 X 10-0 16.5 34.6 3 . 3 X 10-7 1.95 x 10-8 16.0 34.6 ASTO N 28 e.u.; AH“0 (av.) = 34.7 4 kcal. [FeBr*(s) = FeBrs(g)] = 47.1 kcal./mole. See AHOUO(av.) = 59.5 f 6 kcal.
622 635 645 665
AHTO?)
kcal./ mole dimer reaction (b)
59.4 59.4 59.6 59.6
T,OK.
pFeBra/pFerBrc
645 24.6 964 (m.p.)O 1.1 1215 (b.p.)) 1.1 a Reference 2. Extrapolated value, this work.
mately equal partial pressures of monomer and dimer in the saturated vapor over FeBrz(1). This AHTO ref. 1; dimer concentration is somewhpt larger than that estimated as an upper limit (15%) by MacLaren and Gregory from a comparison of their transpirasublimation for Fe2Br4(g)shown in Table 11. The tion and diaphragm measurements in an overlapuncertainties are due mainly to the entropy and ping temperature range near the melting point. ionization cross section terms. The difference in It is interesting to note, however, that, with the aid heats of sublimation for FezBrd(g) and FeBrz(g) of the previously estimated AC, (sublimation) of may be calculated from the slope of the IFeBrt+/ FeBrz(g), the extrapolation of their vapor pressure I F e B r 3 + us. 1/T curve shown in Fig. 1. In this curve obtained from effusion experiments a t low plot IFe2BrBt is used instead of I F a B r 4 + in order to temperatures, where the monomer is the major gain a factor of approximately 1.5 in sensitivity. vapor species, gives a monomer vapor pressure at The difference in heats of sublimation, 14.2 kcal., the melting point which is closer to 50% of the determined by a least squares treatment of the total pressure obtained from their diaphragm measgraphical points is in good agreement, considering urements. The accuracy in calculation of the the uncertainties involved, with the value of 12.4 monomer-dimer ratio in our work is subject to the determined by the third law method. This agree- uncertainty in the heat of dimerization of the ment establishes some degree of confidence in the monomer. With the present results, it is only estimated value for the entropy of dimerization for possible to set the limits of 30-70% for the dimer FeBm(g). concentration a t the melting point of FeBr2 where The data in Table I11 represent an extrapolation the total pressure is taken to be 3 X atof our calculated pressures to higher temperatures mosphere. using a heat of sublimation of FeBr2(g) of 47.1 (4) M . G . Inghrain, R. Hayden and D. Hess, “Mass Spectroscopy in kcal./mole and a heat of fusion of FeBr2(s) of 11 Physic8 Research,” Nat. Bur. Standards Cir. 522, 1952. kcal./mole from ref. 1, a heat of sublimation of (5) I(. K. Kelley, U.S. Bur. Mines Bull. No. 476, 1949.
*
.
