Surface diffusion of hydrogen on platinum was detected by permeation of H2 and inert gases in porous Pt.Below 1 Torr at 60-75°C surface transport was substantial in comparison with gas-phase transport. Surface diffusivities Ds were about 10-6 cm2/sec and the activation energy was 5.7 kcal/mole! Comparison of these and literature values of Ds showed a relation between Ds and the heat of adsorption q. Three classes of behavior were found, and differences were attributed to differences in the type of gas-surface bonding. Numbers m were assigned to each bond type and a general correlation was produced, Da = 0.016•

Surface transport is described in terms of the hopping of adsorbed molecules between adjacent sites of different adsorption strength. The change in surface diffusivity with surface concentration is attributed to a change in the strength of adsorption, as evidenced by a change in the differential heat of adsorption, q, with concentration. The correlating equation predicts that the surface diffusivity varies as exp(~aq/RT), where a is an experimental constant. New data on the flow of C02, S02, and NH3 in porous glass are presented and are correlated successfully by the above equation. In addition, literature data for five other systems are correlated well by this method.

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Rates of flow of pure gases, both those with no adsorption and those with appreciable adsorption, were studied as a function of pressure level, pressure drop, and temperature for flow through 1/2-in.-diameter cylindrical plugs of activated carbon and of unsintered Vycor glass. Adsorption isotherms for the pure gases on Vycor glass were measured over the range of variables covered in the flow studies. A few measurements were made for bulk liquid flowing through a Vycor plug.Permeabilities, which are proportional to the rate of flow per unit of pressure drop, were satisfactorily correlated for hydrogen, helium, argon, and nitrogen by employing existing gas-phase flow theory. Permeabilities considerably larger than the values predicted from the nonadsorbed gas correlation, sometimes more than seventeen times as large, were observed for ethylene, propylene, and isobutane flowing through a Vycor plug. For the hydrocarbon-Vycor systems, permeabilities for vapor flow are as much as sixty times larger than for bulk liquid flow.The unusual flow phenomena for the hydrocarbon-Vycor systems are attributed to a rapid transport in the adsorbed layer. The total transport is treated as being the sum of gas-phase and adsorbed-layer flow. An equation describing adsorbed-layer movement is derived by utilizing a force balance together with thermodynamic principles. The resulting equation has just one empirical constant, and its use requires adsorption-isotherm data. It correlates very well the surface flow rates for the major range of the variables covered in this investigation. Rate measurements were made for adsorbed-lager concentrations ranging from about one tenth of a monolayer up through the capillary condensation region. Deviations in the one constant form of the equation are observed below one tenth of a monolayer. The available literature data on flow in adsorbed layers are reasonably well correlated by the same equation.

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