SURFACE DIFFUSION OF HYDROGEN ON CARBOX
Table I : Assumed Values for the Calculation of Surface Diffusion Quantity
Symbol
Value
Jump distance Frequency Radius of diffusion source Coefficient of proportionality Adsorption a t 300” and 60 cm. Number of diffusion sources
d
3.41 A. 10lSsec.-l 10A. 0.58 1016atoms/cm.* 3 . 1 6 x 1017
P
a a co
n
It can be seen frorn Table I1 that, with those values, a good agreement is obtained between calculated and experimental amounts of hydrogen adsorbed. I n other words, the explanation in terms of surface diffusion can be quantitative. At higher temperatures and longer times, calculated values are systematically higher than experimental values. This effect of “saturation” has been mentioned earlier. It may be due to an interference or overlap between diffusion zones which have been assumed to be separated and independent .
2753
interpreted successfully in terms of surface diffusion. Although it has been possible to propose a value of the surface diffusion coefficient, a quantitative treatment of the data is made very difficult by the number of parameters of the problem. Although there may be other cases where slow adsorption can be explained by surface diffusion from or to active centers, a quantitative explanation may prove even more difficult than for the very favorable case of platinized carbon considered in this investigation. In the determination of surface areas by selective chemisorption, the phenomenon may be quite troublesome.
Discussion R. A. VAN NORDSTRAND (Sinclair Research Co., Houston). Have you studied the reversibility of this chemisorption? It seems that your mechanism consists of readily reversible steps, so that simply by dropping the hydrogen pressure to zero you should be able to desorb and follow the process using the same equations.
s
Conclusion Qualitatively and to a certain extent quantitatively, the uptake of hydrogen by platinized carbon can be Table I1 : Comparison between Calculated and Experimental Amounts of Hydrogen Diffusion on 0.2% Platinum Sample Temp.,
Amount of surface diffusion, CC.of Hz (STP) Caled. Exptl.
O C .
Pressure, cm.
Time, min.
300
60
5 10 50
0.15 0.22 0.59
0.21 0.29 0.60
30
5 10 50
0.10 0.15 0.41
0.15 0.21 0.39
60
5 10 50
0.25 0.35 1.2
0.42 0.58 1.1
30
5 10 50
0.18 0.24 0.87
0.26 0.34 0.64
60
5 10 50
0.49 0.73 2.5
0.63 0.84 1.3
30
5 10 50
0.35 0.52 1.7
0.40 0.55 0 . 9 6 (est.)
350
392
A. J. ROBELL. No, we have not studied the reverse process. The desorption kinetics would not be predicted by the same mathematical solution. The boundary conditions to the unsteady-state diffusion equation would be different; the sourre would be a two-dimensional field of varying concentration, and the sink a small area of essentially zero concentration. We have noted that a n instantaneous reduction in the hydrogen pressure during an adsorption run produces a n immediate diminution in the rate of net adsorption, as predicted from our niodel.
J. W.’ E. COENENS([Jnilever, Netherlands). I n general I understand your wanting to “correct” your experimentally observed adsorption on platinum-carbon by subtracting the amount of gas adsorbed on carbon alone, but should it riot be borne in mind that we are primarily concerned with the direct experimental observation, rather than with a subtraction of the two? This is especially important in the case of the kinetic isotope effect. You actually did appear to observe such an effect with carbon but you found no effect on the carbon-supported platinum. But it seems to me that you cannot say that the kinetic isotope effect is reversed, although your observation is still significant enough. A. J. ROBELL. The experimental facts are ss follows: on the carbon alone, hydrogen adsorbs fsster than deuterium; on the platinized carbon, hydrogen and deuterium adsorb at essentially the same rate. Thus, “net” adsorption, as it is defined here, proceeds a t a faster rate with deuterium thcn with hydrogen. The point that I wish to bring out here with these observations is the following: the “normal” kinetic, isotope effect, i e . , the one in which hydrogen is faster than deuterium, observed in direct adsorption with carbon alone is not observed when platinum is dispersed on the carbon. This implies a different rate-(:ontrolling procese.
Volume 68,Number 10
October, 1964
