The Nature of Weak Chemisorption on Pd

The Nature of Weak Chemisorption on Pd.—Several workers have investigated adsorption and absorption of H on Pd in order to determine the surface areas of both unsupported and supportedpowders. Lynch and Flanagan have presented adsorption isotherms for weak chemisorption on Pd. At 273 K the isotherm shows no tendency to saturate and the excess sorption is 0.34 monolayers. 

The difference between Pd films and powders is notable. Some of the H retained by the powders may be situated at surface imperfections. However, it is also possible that some of the hydrogen is incorporated within the defect structure of the powders, in much the same way as suggested earlier for Pt blacks. Indeed, Aben and Frieske and Wicke have noted that very high temperatures ( 700 K) are required to desorb all the H from Pd blacks. As with Pt blacks the agreement between surface areas derived from H chemisorption and Kr adsorption may be fortuitous. In any case it is apparent that evacuation at T 300 K causes the desorption of some H from the adsorbed monolayer. Consequently, at these temperatures the excess adsorption may actually correspond to readsorption into the original monolayer. 

Recent data of Konvalinka and Scholten may also be interpreted as support for easy hydride formation in small Pd particles. When they expose a Pd sponge to pulses of H at 233 K they observe a kink in a plot of H uptake versus number of pulses, which they assume to be the point at which a monolayer of H is formed. Since the total uptake of H is equivalent to 5.75 monolayers they suggest that there is also rapid absorption into the a phase of Pd/H. However, the maximum solubility of H in the a phase of their sample would only be equivalent to about 1 monolayer. The remaining excess H may be accommodated at structural imperfections, although the enhanced solubility is considerably more than would be expected. It is more likely that some ) -phase hydride is being formed, possibly only in the surface layers of the sponge. Eley and Pearson have also postulated the formation of a. 

Harris, and J. R. Hirst, L Hydrogene dans les Metaux , Tome 1, Editions Science et Industrie, Paris, 1972, p. 1. 

One final important fact which comes from Sermon s data is that at 300 K a monolayer coverage is only completed at a hydrogen pressure of 400 Nm. Data for bulk pd shows that at this temperature and pressure hydrogen will have been absorbed up to PdHo.oo4- Here again absorption and adsorption (9 1.0) seem to occur simultaneously. 

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