Page 176 - Principles of Catalyst Development

164 CHAPTER 7

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Figure 7.24. Hydrogen chemisorption on Pt/ AI 20.
interface with the support. Extrapolation back to zero pressure is necessary
to determine the monolayer.
Second, the stoichiometry of adsorption must be known in order to
calculate surface concentrations. This is extremely difficult to establish.
Hydrogen, for example, adsorbs as M- H species over crystalline planes.
This is confirmed with parallel hydrogen chemisorption and BET measure-
ments on nonsupported nickel. However, the stoichiometry of the bond
M-Hn appears to increase for low coordination sites (see Chapter 3) so
that overall values for very small crystallites may be greater than one. Carbon
monoxide is even more troublesome. Several modes coexist; linear Ni-CO,
bridged, Ni2CO, and subcarbonyl, Ni( CO )2, so that some assumptions are
inherent in its use.(223)
In spite of these difficulties, H2 and CO chemisorption are favorites
for dispersion measurements. Table 7.8 gives typical results(22'l) and Table
7.9 lists other successful applications.

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Figure 7.25. Hydrogen chemi:.orption on Ni/SiO,.