NOTES
Oct,., 1960
Having chosen this structure, the isotope effect then can be explained on the basis of the simple mechanism n--C
03 // !
\ ;
0
\\
+
C-H
H--0
/
+ 0
08
1
\\
L
C-H
‘O---- H-0
/
’
A
which assumes that each of the polymeric ions is formed by the rearrangement of the initial collision complex. The rearrangement always involves the migration or tunnel effect of the hydrogen or deuterium nucleus originally attached to the negative reactant ion to form the more stable and symmetrical double hydrogen-bonded resonance structure. Since the deuterium would migrate more slowly than hydrogen, the collision complex for reactions 3 and 4 would dissociate on the average more often than that for reactions 1 and 2 ; hence, kS = k4 < k1 = kz. On the basis of this hydrogen rearrangement mechanism, the relative abundances of the various polymeric ions for any given mixture become 91:92:93 = a 2 : ( 2- y)ab:(l
- y)b2
where (1 - y) is the isotope discrimination factor, X-3 ’kl. Strong experimental confirmation of this postulated mechanism is presented in Table I where the calculated results were obtained taking y = 0.65, which gives a minimum error for all the mixtures studied. These considerations are equally applicable for a rearrangement of the formate ion prior to formation of the collision complex. TABLEI PERCEUT.ABUNDANCE OF KEGATWE IOKS FROM HCOOHIICOOH MIXTURES(all data taken at -200’) Mixture Mass 45 Mass 46 IICOO- DCOOobsd. obsd.
67 GO 54 47 32
33 40 46
53 68
LMass 91 HCOO HCOOHobsd. calcd.
52 45 42 32 18
Mass 92 HCOODCOOHDCOOHCOOHobsd. oalcd.
Mass 93 DCOO DCOOHobsd. oalod.
5G
40
38
48
46 47
44 48
7 9
6 8
11
52 55
52 52
lG 27
11 15
41 33 18
30
Structural rearrangements of positive ions are well known6 but similar processes of negative ions have not been reported previously. It is also interesting that the “even” negative ion HCOO(6) (a) P. N. Rylander and S. Meyerson, J . Am. Chem. SOC.,78, 5799 (1956); (b) V. Hanus, Nature. 184, 1796 (1959).
1579
appears to react with smaller bimolecular rate constants than are observed for most “even and odd” positive i011s.~ For example, by method5 previously described,’ we find for these reactions from this study and a previous study?
+ HCOOH kl HCOOHCOOHk* HCOO- + N, +CK- + [XOnH]
HCOO-
----f
(where [iYOzH]represents all of the neutrals necessary for a material balance) that kl = L9X lo-’’ and k* = 7.6 X in units of cc. molecule-’ set.-'. The smaller rate constants of these negative ion reactions represent smaller probabilities or steric factors of reaction. This is consistent with the greater structural changes associated with these reactions. (7) T. Pi. Martin and C. E. Blelton, J. Chem. Phys., 32,700 (1960). (8) C. E. Melton and G. A. Ropp, J. $ m . Chem. S a c , 80, 5373 (1958).
CHEMISORPTION AS A PREREQUISITE TO HETEROGEKEOUS CATALYSIS BYR. bf. H. SACHTLER AKD K. H.
DE
Konznklz~ke/Shell-Laboratoraum. AmsteTdam (Shell Research Maatschappzf N. T.) Receaved March 2SS 1960
BOER Inteinahonale
To understand mechanisms in heterogeneous catalysis it must be known whether stable chemisorption of all reactants is necessary or whether reactions occur between loosely adsorbed complexes. For the simple reaction Hg
+ De
2HD
Bonhoeffer, et uLll and Farkasz assumed a dissociative chemisorption of both gases, followed by random desorption of atom pairs. A different mechanism was advocated by Ridea13 and E l e ~ who ,~ considered the reaction between a chemisorbed atom and an impinging molecule. We have approached the problem in a new way, using gold as a catalyst. Gaseous hydrogen molecules are not measurably chemisorbed on gold below 200”. On the other hand, when formic acid decomposes on a gold catalyst the gold surface becomes partly covered by hydrogen atoms during the steady state of the decomposition r e a ~ t i o n . ~Consequently, when a mixture of HCOOH Dzis in contact with a gold catalyst, only the light hydrogen isotope passes through the stage of atomic chemisorption, whereas the heavy isotope is not chemisorbed. We have taken advantage of this particular situation to carry out a number of experiments, the results of which are given below. The gold was
+
(1) K. F. Bonhoeffer and A. Farkas, Z . physzk. Chein., Bl2, 231 (1931). K. F. Bonhoeffer, F. Bach and E. Fagans, sbcd., A168, 313 (1934). (2) A. Farkas, “Orthohydrogen, Parahydrogen and Heavy Hydrogen,” Cambridge Univ. Press, 1935. (3) E. K. Rideal, Proc. Combndoe Phzl. Soc., 35, 130 (1939). (4) D. D. Eley and E. K. Rideal, Proc. Roy. S O C .(London). A178, 429 (1941). ( 5 ) J. Fahrenfort, L. L. van Reijen and W. M. H. Sachtler, “The Mechanism of Heterogeneous Catalysis,” 1960, in press; 2. Elcktrochem, 64, 216 (1960).
NOTES
1580
Vol. 64
the spinel in the paper.' It was found, hom-ever, that such a likeness exists only in first approximation. Among the 27 observed rings only 24 fitted to the theory, giving a = 8.97 A. f 0.5%, the (A) Hz-- Dz +no formation of H D others three rings (220, 331, 664) give the discrep(B) HCOOH $. Dz 4 only Hz and COZare formed; no formation of HD ancy more than 1%. It follows therefrom that (C) HCOOH $. DCOOD +Hz,Dz and HD are formed the crystal lattice is not really that of spinel type. in equilibrium distribution Just for this reason I have used above 27 reflections +, Hz, Df and H D are formed to build the radial distribution curve, according to (D) IICOOD in equilibrium distribution Mackle and Sut,ton3 and obtained from it among The rates of reactions C and D are those of formic others a maximum at r = 1.4 A., which shows that acid decomposition, e. g., 6.4 X molec. site-' there are oxygen molecular ions in the structure. see.-' for COOH. For the constant k, given by: Hence it appears that the surface structure is of k = [HD]2/[[H2] [Dz] , a value of 3.50 =t 0.1 was peroxide type, probably due to the gold, but not of found in all experiments where mixtures of HCOOH spinal type. HCOOD iDCOOD were decomposed. The An additional proof of existence of surface gold theoretical figure for equilibrium, as given by Deroxide was obtained by the study of particular Farkas,* is k t h = 3.52. texture-diagram from the surface of gold heated at II. Hydrogen oxidation at 120". TjOO", or nearly so, in oxygen at normal pressure. It has been found4 that the structure belongs to (E) 2Hz f Oz +very slow formation of HnO hexagonal type with a = 5.28 and c = 6.75 (Tril(F) 2HCOOH 4- 02 -+-fast formation of HZO (G) 2IlCOOEI 02 f excess D2 +fast formation of lat5 obtained from a thin leaf, butfi otherwise in HzO, containing similar conditions, the same period a = 5.28 A.). no DOH or DzO, Of course it was possible to determine the posiexcept traces; no Hz present in tions only of heavy gold atoms, but at the same time the interstices between them were found to be large the gas phase enough to locate the molecules of oxygen with their From i,hese results it is concluded that under the large axes parallel to the sixfold axis of lattice. conditions used dissociative chemisorption of It may be pointed out also that various electron hydrogen is essential for the catalytic mechanism diffraction diagrams arise when condensed gold is of both reactions studied. Moreover, result G heated in oxygen at other from 100 to shows that oxygen quantitatively consumes chemi- 900". Many examples aretemperatures a receiit book.6 given in sorbed hydrogen atoms, whereas the gaseous DZ However, in no case were indications obtained on molecules remain untouched. the spinel or other structures of unnobie metals. It is seen from these considerations that gold is capable of giving surface peroxides after heating ON T H E OXIDATION OF GOLD and that the existence of unnoble oxides IS imBY N. A. SHISHAKOV probable.
used either in powder form or dispersed on a carrier. I. Hydrogen exchange at 150".
+
-+
Instktute far Physical Chemistry, Academy of Science, M O S C OU.S.S.R. ~D, Received April 1. 1960
It has been shown in two recent papers of Carpenter and Clark with co-workers' that different oxides arise on the surface of gold as a result of its interaction with oxygen at low pressures. The authors concluded that these oxides belong to the spinel type with iron, copper and lead cations, which were arisen from contamination of gold. However, the proofs for this statement are quite insufficient. For instance, the authors attribute one of their electron diffraction diagrams to FeO. Fez03with the length of unit cell a = 8.34 A,,but they do not indicate the degree of agreement between observed and theoretical spacings and intensities. The following fact will show that the neglecting of such requirements may lead to quite erronepus results. It wasl shown in our previous communication2 that the surface of gold heated a t 500" in oxygen under normal pressure gives excellent electron diffraction diagrams with a number of rings, listed in this communication. The structure remained unknown, but it may be noted that the diagram is very like to that which was attributed to (1) L. D. Carpenter, D . Clark, W. H. Mair and T. Dickinson, Trans. Faraday Sac., 66, 19:!4 (1959). (2) N. A. Shishakov, J . I'hys. Chem. (Zluss.), 31, 33 (1957).
If. Mackleand L. Sutton, T r a m Faraday Soc., 7 , 691 951 (1977) N. 9.Shishakov Krrstallograph1e (Russ ) , 2, 686 (1967) J. J . Trillat, Sh. Oketani J . de phys. e t rad., 8 , 59, 93 (1937) N. A. Shishakov and V. V. Andreewa, "Oxlde Films on Metals " h a d . Set. U R S S , .Moscow, 1959.
(3) (4) (5) ' (0)
DIFFUSION MEASUREMENTS WITH A DIAPHRAGM CELL BY H. L. TOOR Chemical Engineerana Peportment Carnegze Institute af Trchnolow, Pzltpburgh 19,'Pennaylvanaa Rceetved 9 p r 2 1 11, 1960
In the measurement of diffusion coefficients by the diaphragm cell method, quasi steady-state diffusion is allowed to take place through the pores of a diaphragm. The differential diffusion coefficient to be measured is defined by Fick's first lam j = -D(C)VC
(1)
where j is the flux vector and C the concentration of diffusing substance. The integral diffusion equation for a diaphragm in which the concentratioris at the parallel x = 0 and 2 = L faces' are C1 and Cz, respectively, is normally obtained as follows: It is assumed that (1) the (1) The diaphragm faces are defined by two y-z planes in a rectangular co6rdinate system whose z cobrdinatea are 0 and 1,. Diffueion takes place only through these faces.
