Dispersion by Selective Gas-Chemisorption

Unlike the physical methods of characterisation outlined above, the theory underlying this procedure is very straightforward and the equipment needed is relatively cheap, but the practice is surrounded by pitfalls for the unwary and the interpretation is fraught with difficulties. Starting with a clean surface, one measures the number of molecules needed to form a monolayer just on the metal, and from this number, which is taken to be equal or proportional to the number 

There are various ways in which the monolayer volume can be measured. In the static method, successive small doses of the adsorbate are admitted, and the number of adsorbed molecules after equilibrium has been reached are deduced either gravimetrically (possible with carbon monoxide, difficult with hydrogen) or volumetrically from the residual pressure or by some other technique such as NMR or XANES. The procedure is repeated until no further uptake occurs, when the monolayer capacity will be known. If chemisorption on the metal is strong and 

Specific to the metal, the adsorption isotherm will show an initial rapid increase of uptake with pressure, followed by a linear region of low slope. The monolayer capacity is often reported (when anything at all is said) as the intercept at zero pressure obtained by extrapolation of the linear part. Alternatively, and perhaps better, the results are plotted according to the one of the linearised forms of the Langmuir equation from which the monolayer volume can be obtained. 

It is claimed that measurement of the saturation amounts of hydrogen, oxygen and carbon monoxide can quantity the relative contributions of the three low index planes exposed by palladium particles this however assumes that all adsorbing atoms belong to one or other of these planes.  

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