HYDROGEN-DEUTERIUM EQUILIBRATION OVER PALLADIUM HYDRIDE
hour or two after the solution had been prepared. The peak corresponding to the enol form decreased in intensity while that corresponding to the keto form increased in intensity. Preliminary kinetic study of the enol-keto conversion in carbon tetrachloride showed it to be approximately first order in the concentration of
2905
enol, with a half-time at 34” of about 10 min for a solution containing 1.54 M concentration of methanol. Acknowledgments. This work was done under Contract No. AT-(30-1)-1922 with the U. s.Atomic Energy Commission, and under an NSF Summer Faculty Participation Program.
Hydrogen-Deuterium Equilibration over Palladium Hydride
by R. J. Rennard, Jr., and R. J. Kokes Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland (Received March 7 , 1966)
21218
Equilibration of a 50 :50 mixture of hydrogen and deuterium over palladium hydride has been studied as a function of composition at -195 and -183”. Sorption of hydrogen and deuterium occurs at these temperatures, but exchange of gas phase deuterium with palladium hydride (or vice versa) does not. The rate of equilibration decreases with increasing hydrogen content of the catalyst. It is concluded that once hydrogen adsorption occurs, exchange or incorporation follows and that the relative rates of these two processes are independent of hydride concentration in the palladium.
Introduction Palladium hydride’ is a general term applied to a nonstoichiometric series of compounds ranging in gross composition from roughly H/Pd = 0.0 to H/Pd = 0.8.’ Above 310°, palladium hydride forms a single phase; below 310”. it forms two phases. At room temperature, the pure a phase exists in the approximate range, 0 < H/Pd < 0.06; in the range 0.06 < H/Pd < 0.55, a mixture of a and P phases is found; only the pure 6 phase is found for H/Pd > 0.55. (These boundaries depend somewhat on preparative conditions.) Despite the complexity of this system, the magnetic susceptibility2 is a linear function of concentration that approaches zero at H/Pd = 0.66; similarly, the electrical resistivity3 is a linear function of concentration up to H/Pd = 0.80. These linear relations hold through the two-phase region. Thus, hydrogen behaves approximately like an alloying metal that fills up the holes in the d band at the composition PdH,,,,. Several have observed that hydride
formation affects the activity of the catalysts both for the ortho-para conversion and ethylene hydrogenation. Of these reactions, the former is by far the simpler, but it can proceed primarily by the magnetic conversion and, hence, shed relatively little light on the chemistry of the hydrogen activation process.* Accordingly, we have selected the hydrogen-deuterium equilibration for study as a function of the hydrogen content of palladium hydride. (1) D. P. Smith, “Hydrogen in Metals,” University of Chicago Press, Chicago, Ill., 1948. (2) C. Kittel, “Solid State Physics,” John Wiley and Sons, Inc., New York, N. Y., 1956, p 334. (3) R. E. Norberg, Phys. Rev., 86, 745 (1952). (4) A. Farkas, Trans. Faraday SOC.,32, 1667 (1936). (5) A. Couper and D. D. Eley, Discussions Faraday SOC.,8, 172 (1950). ( 6 ) R.I. Kowaka, J . Japan Znst. MetaEs, 23, 625 (1959). (7) R. J. Rennard, Jr., and R. J. Kokes, J. Phys. Chem., 70, 2543 (1966). ( 8 ) G. C. Bond, “Catalysis by Metals,” Academic Press Inc., New I‘ork, N. Y., 1962.
Volume 70, Number 9 September 1966
R. J. RENNARD, JR.,AND R. J. KOKES
2906
Experimental Section Palladium was prepared by a modification' of the procedure used by Gillespie and H a L 9 This yielded a palladium sponge with a surface area of 0.48 m2/g. Prior to each kinetic run, the palladium was heated for 16 hr in 200 mm of hydrogen a t 400" and degassed a t 450" for 30 min. Then the sample was cooled in helium to -78" and evacuated. The hydride was prepared by sorption of a measured amount of gaseous hydrogen; at -78" sorption was rapid, irreversible, and complete for compositions up to PdHo.a. After the hydride was formed, the sample was cooled in helium to -195" (or -183"), evacuated, and a 50:50 mixture of hydrogen-deuterium was admitted to the reaction flask. From time to time, a small sample of the reactant gas was withdrawn and analyzed mass spectrographically. From the analysis and mass balance it was possible to compute not only the amount of HD produced but also the amount of sorption of hydrogen and deuterium that accompanied the equilibration reaction. All equilibration runs were carried out in a 30-cc conical reaction flask with the catalyst at the bottom. The catalyst sample (40-60 mesh) consisted of 320 mg of palladium admixed with 1.5 g of crushed Vycor (40-60 mesh). In studies of adsorption alone, a 5-g sample of palladium in a tightly packed tube was used. These results agreed reasonably well with results obtained during kinetic experiments.
Results Exchange Experiments. At - 195", no exchange occurred in 3 hr between palladium deuteride and gaseous hydrogen or gaseous deuterium and palladium hydride. At -183", no exchange between gaseous deuterium and palladium hydride was detected. Adsorption Experiments. Figure 1 shows a plot of the hydrogen uptake at 200 mm by pure palladium and PdHo.23at -195". It is clear that bulk reaction occurs even at - 195" for the amount of sorbed hydrogen greatly exceeds a monolayer; after 300 min, the uptake of hydrogen for both samples is of the order of 30 monolayers. (Results of Knor, Ponec, and Cerney'O can be interpreted in terms of bulk sorption a t - 195O.) Initially, the rate of sorption for the pure palladium is lower than that for PdHo.23, but in time, the rate for pure palladium becomes nearly linear and greater than that found for PdHo.23. I n what follows we shall term the initial nonlinear portion of this curve the induction period. Deuterium sorption by palladium a t -195" is a t least an order of magnitude slower than that for hyThe Journul of Physical Chemistry
0.B 0.6 0.4
0.2
0 0
60
I20
180
240
300
Time (min.)
Figure 1. Sorption of hydrogen at -195": open circles, pure palIadium; solid circles, PdHa.23.
drogen. The induction period is correspondingly longer. Figure 2 shows a plot of the hydrogen uptake a t 200 mm by pure palladium and PdHo.zaat -183". The rate is an order of magnitude greater than at -195" and the induction period is much shorter. From these data it is much clearer that the sorption rate for pure palladium eventually becomes greater than that of PdHo.23. At -183", the rate of deuterium adsorption is about 30% of that for hydrogen. Comparison of rates at - 195 and - 183" shows that the activation energy for sorption is of the order of 2-3 kcal. Extrapolation of these data shows that the rate should be about lo6 times faster a t -78". This is in agreement with the experimental observation that the half-time for sorption with an initial pressure of 200 mm of hydrogen is of the order of 5 sec at -78" compared to an estimated lo4min at - 195". Equilibration. At - 195", the rate of H D formation from a 50 :50 H2-Dz mixture is quite low ; for example, after 300 min exposure to the most active catalyst, about 1% of the gas phase is HD. As with sorption, there is a prolonged induction period for pure palladium, but none is evident for th'e hydrides. Equilibration rates were estimated from the straight-line portion of the curves of HD formation us. time; these rates, relative to that for pure palladium, are plotted in Figure 3. (9) L. J. Gillespie and F. P. Hall, J. A m . Chem. SOC.,48, 1207 (1926). (10) Z. Knor, V. Ponec, and S. Cerney, Kinetics CataEysis (USSR), 4, 437 (1963).
HYDROGEN-DEUTERIUM EQUILIBRATION OVER PALLADIUM HYDRIDE
2907
P
70 60
n I ~~
50
v) Q
0
+
Q
40
c1
/"
P' / ,*
'*,/
3.0 .-
3 N
r
0
25
75
50
0
125
100
25
T i m e (min.)
50
75
IOC
Time (min.)
Figure 2. Sorption of hydrogen at -183': open circles, pure palhdium; solid circles, PdHo.28.
Figure 4. HD formation a t -183': open triangles, pure palladium; solid circles, PdHo.oea; open circles, PdHo .za3; solid triangles, PdHo.ca.
60 78-K
\
/*
501
0 90'K
40
\
\
\
t 01
0
' I
I
I
I
0.2
0.4
0.6
I
H/Pd Figure 3. Relative equilibration rate us. composition: solid circles, - 183'; open circles, -195".
0 0
Figure 4 shows a plot of the amount of gaseous HD formed a t - 183" as a function of time when a catalyst with the indicrited composition is exposed to a 50:50 H2-Dzreactant. I n these runs, after 300 min, as much as 10% of the gas phase was HD. Comparison of the rates over pure palladium at - 183 and - 196" suggests that the activation energy for the equilibration is of the order of 2-3 kcal, a value comparable to that for sorption. ,4t -183", the rate of sorption of hydrogen and deuterium from the reactant mixture could be obtained from the analysis and mass balance. The accuracy of these data was not as good as that shown in Figures
I
I
I
I
15
30
45
60
75
p moles HD Figure 5 . Deuterium uptake us. HD formation at - 183" : solid circles, pure palladium; open triangles, PdHo.oss; open circles, PdHo,233; solid triangles, PdHo,ros.
1 and 2, but separate check experiments showed the rate data were reliable to about 10%. The hydrogen uptake curves were similar to those shown in Figure 2 ; the deuterium uptake curves were also qualitatively similar except for the curve for pure palladium which showed a more pronounced induction period than that in Figure 2 . Quantitatively, the curves for deuterium Volume 70, Number 9 September 1966
R. J. RENNARD, JR., AND R. J. KOKES
2908
differed from those for hydrogen; the rate of deuterium uptake was only 25 to 30% that found for hydrogen. Rates of deuterium sorption for the various catalysts a t - 183" were clearly similar to the rates of equilibration. This similarity is illustrated by Figure 5, which plots for each catalyst the amount of deuterium sorbed (as H D or D2) vs. the amount of H D formed for each of the catalysts studied. I n spite of the fact that the curves for equilibration (Figure 4) differ greatly in magnitude and shape, the plot in Figure 5 is represented by a single straight line through the origin within the experimental error. For runs at - 183", the hydride composition changed significantly during the course of the reaction because of sorption. I n Figure 3 we plot the relative rate of HD formation (from the straight line portion of Figure 4) us. the initial H/Pd ratio. At the end of these experiments the ratio (H D/Pd) was 0.02 to 0.04 higher than a t the beginning of the experiment.
+
Discussion Presumably, hydrogen adsorbed on palladium is in atomic form, and we can express the fate of an adsorbed deuterium atom, D*, by the equations
D* -% D(bu1k)
(1)
k2
D*
+ H* -% HD(g)
(3)
The lack of exchange with palladium deuteride shows reaction 1 is irreversible; the reverse of reaction 3 is unlikely to be important when the amount of H D formed is low. With the steady-state approximation we obtain dD* -dt
=
k,(D*)
+ '/zkz(D*)'
-
'/zk-ZDz
+ k3(D*)(H*) = 0
wherein the rate constant, k-z, probably depends on the surface coverage. The results show that we may write klD* = pk3(D*)(H*),where p is the slope of the
The Journal of Physical Chemistry
line in Figure 5. When this is substituted in the above equation, we find dHD ~dt
- k3(H*)(D*)
=
ka(Dz)
- kz(D*)'
+ 1)
Figure 3 gives plots of the rates of H D formation relative to that for pure palladium as a function of hydride composition a t -183 and -195". If, and only if, p is a constant a t all compositions, this plot also represents the net relative rate of formation of adsorbed deuterium atoms, i.e., the rate of deuterium activation. At -183", /3 is very nearly a constant (Figure 5 ) ; even at -195", the less accurate adsorption data suggest /3 is a constant. Accordingly, Figure 3 suggests that the rate of deuterium activation approaches zero near H/Pd = 0.6. The susceptibility also decreases linearly2 with H/Pd and becomes zero a t H/Pd = 0.66. Thus, it is clear that the change in catalytic activity parallels the change in magnetic susceptibility. In this concentration range (0 < H/Pd < 0.66) phase changes occur, the lattice parameter changes, and the holes in the d band are filled. No doubt all these factors influence the susceptibility and catalytic activity. It seems clear, however, that the simplest correlation is with the holes in the d band; as the d bands are filled, both the susceptibility and the activity decrease. The constancy of /3 (= kl/k3H*) implies that H* is sensibly constant in the concentration range studied. This could be achieved if, due to the relative isotope effects for adsorption and bulk diffusion, the surface is sparsely covered with D* and nearly covered with H*. If this be so, the rate-determining process would be the reverse of reaction 2. (The forward reaction would be insignificant.) I n this event, the similarity of the observed activation energies for sorption and exchange is to be expected. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
