Heterogeneity on Oxide Surfaces

The surface laid bare by the desorption apparently remains bare at the higher temperature, although adsorption proceeds at this higher temperature, presumably therefore on other areas of the total surface. When the 

Adsorption rate curve on raising and lowering temperature (hydrogen on ZnO I). 

This phenomenon of rapid desorption on raising the temperature followed by a slow readsorption on other areas was consistently observed 

Adsorption isobar (hydrogen on ZnO III). Roman numeral indicates order of run. 

That the adsorptions involved were actually due to chemisorption was indicated by measurements of nitrogen adsorption on the zinc oxide surfaces studied. These measurements showed that already at 56° C. the van der Waals adsorption of nitrogen was too small to measure on zinc oxide surfaces with surface areas by the BET method in the neighborhood of 5-25 sq. meters/g. The van der Waals adsorption of hydrogen would be still smaller and lower by one or more orders of magnitude than the quantities of gas involved in the desorption and readsorption processes.  

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