NOTES
3499
Table I: Products of Reaction of Acetyl and Methyl Radicals with KO Initial N O pressure, Torr
---___-
r
2.5 2.4 6.7 6.3 8.7 7.3
10.7 10.7 31.0 31.0 75.0 77.0
~ 0 . 4 -0.4
0.02 0.02 0.88 0.69 0.53 0.71
NO.1 NO.1
Nil Nil
on total acetyl radicals, is about 1.5%; this is a minimum figure since some photolysis of methyl nitrite and nitrate occurs. Methyl acetate is a major product and is only suppressed at nitric oxide pressures in excess of 31 Torr, in general agreement with Allen and Bagley. However, we find methyl nitrite but not formamide, as a major product; the absence of nitrite in Allen and Bagley’s work is difficult to understand since nitrate was found, and for methoxy radicalsj7k,/ks = 1.6. Reaction 9, proposed by Allen and Bagley, to explain the formation and persistence of methyl acetate at nitric oxide pressures of up to 50 Torr, seems unlikely. Of the two possible radical routes to methyl acetate
+ NO2 +CH3C02. + NO CH3COz + CH3 +CH3COzCH3 CH3CO + C H 3 0 . +CH3C02CH3
CH30
*
*
(15)
(10)
(11)
reaction 10 seems improbable since Rembaum and Sawarcs found no methyl acetate in the gas-phase pyrolysis of diacetyl peroxide. At 10.7 Torr of nitric oxide, acetone is clearly formed by combination of methyl and acetyl radicals CH3
+ CH&O +CH3COCHs
2CH3C0
-
CHa
- + CHaGO
CzHG
(CH3C0)2
A
CHsCOCH3
0.18 0.18 0.22
0.21 0.05
CHsCOCHs
CHaONOz
0.25 0.20 0.02
Trace Trace 0.12 0.18 0.18 0.15
0.02
0.02 0.02
0.05
bination of methoxyl radicals12 (1012.2mol-’ cm3 sec-’) gives kll = mol-’ cm3 sec-’. On this basis the ratio of the rates of formation of acetone and methyl acetate and its variation with nitric oxide pressure are determined by [CHs.]/ [C?&O-]. Since kl < 1Ol1 mol-’ cm3 sec-’, allowing for falloff on the basis of Christie’s result^,'^ and12 k7 = 109.6mol-’ cm3 sec-’, [CH,.] will decrease more rapidly than [CH30 ] with increasing nitric oxide pressure. Acetone formation is therefore more easily suppressed than is methyl acetate; it is not necessary to invoke a nonradical mechanism for formation of the latter. 0
(7) G. Baker and R. Shaw, J . Chem. SOC., 6965 (1965). (6) A. Rembaum and M. Szwarc, J . Amer. Chem. Soc., 76, 1965 (1954). (9) A. Shepp, J . Chem. Phys., 24, 939 (1956). (10) S. W. Benson, “Thermochemical Kinetics,” John Wiley & Sons, Inc., New York, N. Y., 1968, pp 200,204. (11) A. F. Trotman-Dickenson, “Gas Kinetics,” Butterworth and Co. Ltd., London, 1955, p 111. (12) P. Gray, R. Shaw, and J. C. J. T h y m e , Progr. Reaction Kinetics, 4 , 83 (1967). (13) M,I. Christie, Proc. Roy. Soc., A249, 246 (1958).
(12)
since the results at higher nitric oxide pressures show that only a small fraction of the acetone comes from direct photolysis of biacetyl. There would therefore appear to be no compelling reason to exclude reaction 11 as the source of methyl acetate, unless kll is considerably less than k12. Application of the geometric mean rule to the systems A
-
----
Products, pmo-l CHsONO CHsCOzCHs
co
N 2
Production and Reactivity of Carbon-11 in Solid Compounds’ by Hans J. Ache and Alfred P. Wolf Chemistry Departments, Virginia Polytechnic Institute, Blacksburg, Virginia i.4061, and Brookhaven National Laboratorv, Upton, New York 11973 (Received April 16,1969)
(13) (14) (12)
takinge k13 = 1013.3and JCl4 = 1012.9mol-’ cm2 sec-1 (the latter is estimated taking So = 91.6 and 63.5 e.u., respectively, for biacetyP and acety1,’O and’’ L14= 1015e7sec-l) gives klz = 1013.2mol-’ cm3 sec-’. A similar treatment for the combination of methoxyl and acetyl radicals, using the estimated value for the recom-
The reactions of energetic carbon atoms produced by nuclear reactions have been studied in numerous organic systems. Results and postulated mechanisms are the subject of various review^.^-^ Only a few studies (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) A. P. Wolf, Advan, Phys. Org.Chem., 2, 202 (1964). (3) C. MacKay and R. Wolfgang, Science, 148, 899 (1965).
Volume 78, Number 10 October 1989
NOTES Table I : Product Activities in Salt Irradiations Compd
NaF
NaCl
NaBr
Nuclear Process
Nl*(p,a)Cll
N14(pJa)C11
Nld(p,cu)Cll
Postirradirttion treatment
Dissolved in water Annealed for 20 min at 250°, dissolved in water Dissolved in water Annealed for 20 min at 250' dissolved in water Dissolved in water
5% of total carbon-I1 activity produceda ni-Otherscos CH4
C
co 24.5
5.0
15.0
16.1
15.4
28.8
34.7
23.3
Traces
Traces
36.7
63.3
3.5
3.0
88.0
CHaF CHzF2 CHFa CF4 CHaF CHzFa CHFa CF4 CHaCl CHiC12 CHsCl CClr Traces
Remarks
21.0 9.1 3.4 14.0 15.6 9.0 5.0 5.1 15.5 6.2 4.4 15.2
Only traces of C11 in aqueous solution
Only traces of C11 in aqueous solution Only traces of C11 in aqueous solution
CHsBr CH2Brs CHsBr CBr4
These percentages represent the averages of two to five runs.
Table 11: Product Activities in Salt Irradiations, Solvent Effects Nuclear process
Postirradiation treatment
NaCl
Na(p,x)C11
NaBr
Na(p,x)Cll
A1 metal
Al(p,x)C"
Dissolved in water Partially dissolved in liquid NH3 Partially dissolved in liquid CHaNH2 Dissolved in NaOH Dissolved in HCl Dissolved in NaOH Dissolved in HISO,
Compd
N14(p,a)C11
CllO
CI'H4
CllOB
69.5" 27.6"
5.5" 72.9"
25.0"
19. 4a
80.6"
9 None None None
91 100 100
140
None None None None
Remarks
I n % of total gaseous carbon-11 activity I n % of total carbon-11 activity produced, only traces of C11 were detectable in the aqueous solutions
a Numbers represent percentages based on the carbon-11 activity detected in the analysis of the three gases C"0, C11H4, and Cl1OzJ their total yield being set equal to 100. No attempt was made to detect halogenated compounds or to determine the total carbon-11 activity produced in these runs.
have concerned themselves with the fate of an energetic carbon atom in an inorganic solid matrix.0-12 The most favorable systems in which it may be hoped that the detailed relation between dislocations, point defects, and the chemical changes accompanying the reactions of the energetic carbon-11 can be established are the simplest ionic solids, such as the alkali halides, and perhaps metals, such as aluminum. Two different methods have been used to introduce carbon recoil species into the solid alkali-halide or aluminum samples. The first one utilized the Na(p,x) C11 reaction which was attainable by the use of the 3-GeV proton beam of the BNL cosmotron. A total of The Journal of Physical Chemistry
101a-1014 protons were allowed to traverse the target for a 40-min period. In the second method, solid sam-
(4) R.Wolfgang, Progr. Reaction Kinetics, 3, 97 (1965). (5) R.Wolfgang, Ann. idev. Phys. Chem., 16, 15 (1966). (6) P. E. Yankwich and W. R. Cornman, J . Amer. Chem. Soc., 78, 1660 (1956). (7) P.E.Yankwich and J. D. Vaughan, ibid., 76, 5851 (1954). (8) P.E.Yankwich and W. R. Cornman, ibid., 77,2096(1965). (9) (a) P. E.Yankwich, J. Chem. Phys., 15, 374 (1947); (b) P. E. Yankwich and P. J. Marteney in "Chemical Effects of Nuclear Transformations," Vol. 11, IAEA, Vienna, 1965,p 81. (10) (a) J. G. Kuhry and J. P. Adloff, Bull. SOC.Chim. France, 2402 (1967); (b) J. G.Kuhry and J. P. Adloff, ibid., 3414 (1967).
NOTES ples were surrounded by a nitrogen atmosphere in an aluminum tank. The solid sample was consequently bombarded with carbon-11 recoils produced via the K14(p,~)C11 reaction using 10-AleV protons from the BNL 60-in. cyclotron. The recoil range of the 2.1MeV (max) carbon-11 atom can be as high as 5-10 p, depending on the target material used. The proton beam current was 1 FA; exposure times varied from 10 to 60 sec. After the irradiation the samples were dissolved without exposing them to air. The solvents and solutions used are shown in Tables I and 11. In each case, the resulting solution was flushed with a carrier gas mixture composed of CHI, CO, and COz. The gases were collected in a storage bulb from which aliquots were taken and analyzed for carbon-11 by the usual radiogas chromatographic method^.'^!^^ The carbon-11 activity in the remaining solution was measured by scintillation counting. Solvent, in the case of NH3 or CH3NH2,was flash-evaporated by very rapid heating of the solution. The gas was transferred to a storage bulb, from which aliquots were taken for analysis. Some alkali halide samples were thermally annealed, directly after bombardment with carbon-1 1 atoms, a t temperatures of 250' for 20 min. Carbon-11 activity determinations before and after the annealing showed that no carbon-11 activity was lost during the annealing period. Table I gives a representative selection of the percentages of carbon-11-labeled compounds obtained from alkali halides after dissolving the halide in water. The presence of six different reaction products suggests that several carbon-1 1-containing species must exist in the crystals. These can be cationic or anionic as well as neutral carbon species. In addition the observed halogenated hydrocarbons obtained upon dissolving the crystals in water suggest the existence of species such as CllC1, C1'C12, etc., in the crystal. This is especially interesting in the light of recent hot-atom studies in alkali chlorides which showed no direct evidence of [S35C1] or [P32C1] formation following the C13j(n,p) S35l5 or C13j(n,a)P3* reactions. It is reasonable to assume that the halogenated carbon-11 species are produced in hot reactions since the reaction of a 3P0 thermal carbon atom within a (e.g.) sodium chloride crystal to give CC1 is not energetically favorable. Of interest is the increase observed in the yields of the halogenated hydrocarbons as one goes from NaBr to S a F . Whether this reflects the probability of formation of the various halogenated precursors in the hot reaction in the crystal or the reactivity of the trapped carbon-11 species with water during dissolution is as yet undecided. Thermal treatment of the bombarded alkali halide sample results in a reduction of the halogenated C"-labeled products; ie., thermal annealing seems to cause the halogens to reoccupy their original lattice sites under carbon-halogen bond break-
3501 age. One would expect the rate of this process to be, in the main, a function of the lattice energies involved, a fact supported by the results in Table I, which show that the precursors of the fluorinated hydrocarbons are less easily converted to the precursors of C"H4 than are those of the chlorinated hydrocarbons. A shift in the relative yields of the higher halogenated to the less halogenated products following annealing, as in the case of RaF, further indicates a stepwise "C-halogen bond scission." In analogy with the existence of "anionic" carbon in various carbides, which yield methane when dissolved in water, the precursor of methane-C" in these experiments can also be suggested to be a negatively charged carbon atom. This is supported by the fact that in systems which are initially hydrogen and carbon free, such as A1 metal, KaCl, and NaBr, the major fraction of the radioactive carbon, indeed all in the case of A1 metal, can be found in the form of methane-C" (cf. Table 11) when dissolved under reducing conditions. These conditions include the dissolution of AI in acids and bases, as well as the dissolution of alkali halides in liquid ammonia and liquid methylamine. These results also indicate that at least some of the C"0 and C"OZ is not formed as such by reaction with oxygen trapped in the lattice. It can be seen from Table I (NaC1 and NaF) that thermal annealing at a temperature where both C"0 and C"0, are stable causes a marked change in their relative yields. Thus the precursors of C1'0 and C"OZ undergo changes analogous to those yielding methane-C" and other compounds. The annealing work suggests an hypothesis that reactive intermediates consisting of moieties with weak bonds from carbon to halogen or halogens, etc., break down on heating to give some trapped and isolated thermal carbon atoms. Dissolution in a dense medium such as water would then increase the yield of the more exoergic product because of the enhanced probability of deexcitation, thus explaining the favoring of the increase of CllOz and the decrease of C'lO on annealing. Work on these crystals is continuing in the hope of determining the various states of combination of the carbon atom prior to dissolution in various solvents. More detailed theoretical studies of the possible reactions of 3P0carbon atoms and corresponding ions within the crystal and with a variety of solvents will aid in this (11) L.T.Sharman and K. T. McCallum, J. Amer. Chem. Soe., 7 7 , 2989 (1955). (12) F.S. Rowland and W. F. Libby, J. Chem. Phys., 21,1493 (1953). (13) G.Stocklin, F.Cacace, and A. P. Wolf, 2.Anal. Chem., 194,406 (1963). (14) H.J. Ache and A. P. Wolf, J . Amer. Chem. SOC.,8 8 , 888 (1966). (15) See for references: A. G. Maddock and R. M. Mirsky, "Chemical Effects of Nuclear Transformations," Vol. 11, IAEA, Vienna, 1965,p 41. (16) See for references: J. Cifka, "Chemical Effects of Nuclear Transformations," Vol. 11,IAEA, Vienna, 1965,p 71. Volume 73. Number 10
October 1060
NOTES
3502
approach. The nature of the hot reactions leading to these stable and unstable intermediates may then hopefully be elucidated following the results of these studies.
Analysis of Isotherms w i t h Coverage-Dependent Heats of Chemisorption by T. Biegler and R. Woods CSIRO Division of M6neral Chemistry, Port Melbourne, Victoria, 6,807,Australia
(Received M a y 6,1989)
Of t'he adsorption isotherms which explicitly allow for a coverage-dependent heat of adsorption, those which assume a linear variation with coverage are probably the most important for chemisorptive processes because of the frequent experimental observation of this type of behavi0r.l I n deriving these isotherms, it is usual to assume that free energies and enthalpies of adsorption change in the same way with coverage and to insert one of the condition9
or
into the Langmuir equation. Equation 1 states that the standard free energy of adsorption, AGO,, at any given value of coverage, 8, is a linear function of 0 characterized by the constant fe. This equation, which is essentially a statement of the experimental observations, has been justified on the basis of several models including surface heterogeneity, particle interactions, and induced heterogeneity. Equation 2 is based on a particular modelav4of a uniformly heterogeneous surface which consists of a large number of patches of equal area but different adsorption energies; the adsorption energy varies hy equal increments between successive patches. In this relationship, s is the fractional number of patches which have adsorption energy equal to or greater than AGO,. Since both e and s are quantities varying between limits of 0 and 1, eq 1 and 2 are identical in mathematical form and their past use has often contained the implication that they are exactly e q ~ i v a l e n t . ' ~ In ~*~ this communication we shall show that there are significant differences between these equations which lead to diff erences5,' between the isotherms derived from them. It will also be shown that the rate of change of free energy with coverage gives f vaIues which differ from those obtained from the logarithmic Temkin isotherm. The Journal of Physical Chemistry
Isotherm Derived from Equation 1 Insertion of the free energy of adsorption from eq 1 into the Langmuir isotherm gives
where aoe = exp (- AGoe=o/RT) and p is the adsorbate pressure. This isotherm is generally referred to as the Frumkin isotherm, particularly in the electrochem ical literature.6 It is frequently statedS~*~g that the term e/(l - 6') can be neglected in comparison with the exponential term when 0.2 < e < 0.8 and the isotherm is then simplified to
1
0 = - ln(u"ep)
(4)
Sa
If this equation, which has the form of the logarithmic Temkin isotherm, is applied to experimental results, plots of 0 vs. In p will be expected to have a slope of f6-l. However, as we have recently shown,7 t,he approximation leading to eq 4 is not a good one and the slope of eq 3 is, in fact, given by
d0 d In p
e(i - e)
=?
1
+ feO(l
-
8)
At intermediate coverages, this slope is close to (f, 4)-l since, for not too small values of fe, the isotherm is reasonably linear in this range and has a slope close to that at 0 = 0.5.
+
Isotherm Derived from Eq 2 Insertion of the free energy of adsorption from eq 2 into the Langmuir isotherm and integration over all patches gives the complete Temkin isotherm3e4
where uos = exp (-AGo,,o/RT). For intermediate coverages, 0.2 < 8 < 0.8, this equation is usually simplified1t3s4to give (1) D. 0. Hayward and B. M. W. Trapnell, "Chemisorption," Butterworth and Co., Ltd., London, 1964. (2) The positive sign is used in these equations in accordance with the convention that A G O is a negativequantity; by this definition, numerical values of AGO decrease with coverage for positive values off. (3) 11.I. Temkin, Zh. Piz. Khim., 15,296 (1941). (4) S. Brunauer, K. S. Love, and R. G. Keenan, J . d m e r . Chem. SOC., 64, 751 (1942). ( 5 ) E. Gileadi and B. E. Conway, "Modern Aspects of Electrochemistry,'' No. 3, J. O X . Bockris and B. E. Conway, Ed., Butterworth and Go., Ltd., London, 1964, Chapter 6. (6) E. Gileadi, "Electrosorption," E. Gileadi, Ed., Plenum Publishing Corp., New York, N, Y., 1967, Chapter 1. (7) T. Biegler and R. Woods, J.EZectroanaZ. Chem., 20, 347 (1969). (8) B. E. Conway, "Theory and Principles of Electrode Processes," Ronald Press, New York, N. Y., 1965, Chapter 6. (9) 6. Srinivasan, H. Wroblowa, and J. O'M. Bockris, A d z a n . Catal., 351 (1967).
